can't join medium sized federated rooms

*This issue was originally created by [**@rbns**](https://github.com/rbns) at .*

### Background information

- **Dendrite version or git SHA**: 0.6.2
- **Monolith or Polylith?**: monolith
- **SQLite3 or Postgres?**: postgres
- **Running in Docker?**: no
- **`go version`**: 1.16.10
- **Client used (if applicable)**: element-web

### Description

- **What** is the problem: fails to join #go-lang:matrix.org and #dendrite:matrix.org
- **Who** is affected: single user on my dendrite test server.
- **How** is this bug manifesting: join event is propagated to remote servers (user shows up), but join is never completed
- **When** did this first appear: -

### Steps to reproduce

- new install of dendrite 0.6.2 with empty db
- attempt to join #go-lang:matrix.org and #dendrite:matrix.org
- spinning forever in element-web, new join attempt says join is in progress (while events are still processed) or starts new failing attempt.

```
time="2022-02-06T13:51:36.874685465Z" level=info msg="User requested to room join" func="github.com/matrix-org/dendrite/roomserver/internal/perform.(*Joiner).PerformJoin" file="github.com/matrix-org/dendrite/roomserver/internal/perform/perform_join.go:59" room_id="#dendrite:matrix.org" servers="[]" user_id="@rbn:dendrite.collec.tf"
time="2022-02-06T13:53:10.280276162Z" level=info msg="Checking event signatures for 7444 events of room state" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:515" context=missing
time="2022-02-06T13:54:30.808965085Z" level=warning msg="Failed to request keys from fetcher" func=github.com/matrix-org/gomatrixserverlib.KeyRing.VerifyJSONs file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:263" context=missing error="gomatrixserverlib: unable to lookup server keys: Post \"matrix://matrix.org/_matrix/key/v2/query\": dial tcp: i/o timeout" fetcher="perspective server matrix.org"
time="2022-02-06T13:54:36.464257570Z" level=error msg="Failed to fetch key for server" func="github.com/matrix-org/gomatrixserverlib.(*DirectKeyFetcher).FetchKeys.func1" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:497" context=missing error="Post \"matrix://dendrite.rom.dog/_matrix/key/v2/query\": dial tcp 129.153.209.212:8448: connect: no route to host" fetcher=DirectKeyFetcher
time="2022-02-06T13:54:36.464361168Z" level=warning msg="Failed to retrieve any keys" func=github.com/matrix-org/gomatrixserverlib.KeyRing.VerifyJSONs file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:268" context=missing fetcher=DirectKeyFetcher
time="2022-02-06T13:57:24.144458659Z" level=warning msg="Failed to request keys from fetcher" func=github.com/matrix-org/gomatrixserverlib.KeyRing.VerifyJSONs file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:263" context=missing error="gomatrixserverlib: unable to lookup server keys: Post \"matrix://matrix.org/_matrix/key/v2/query\": dial tcp: i/o timeout" fetcher="perspective server matrix.org"
time="2022-02-06T13:58:03.495808786Z" level=warning msg="Failed to retrieve any keys" func=github.com/matrix-org/gomatrixserverlib.KeyRing.VerifyJSONs file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:268" context=missing fetcher="perspective server matrix.org"
time="2022-02-06T13:58:24.120391769Z" level=error msg="Failed to fetch key for server" func="github.com/matrix-org/gomatrixserverlib.(*DirectKeyFetcher).FetchKeys.func1" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:497" context=missing error="Post \"matrix://testuser-dendrite.duckdns.org/_matrix/key/v2/query\": dial tcp 78.46.90.98:8448: connect: connection refused" fetcher=DirectKeyFetcher
time="2022-02-06T13:58:24.120532855Z" level=warning msg="Failed to retrieve any keys" func=github.com/matrix-org/gomatrixserverlib.KeyRing.VerifyJSONs file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/keyring.go:268" context=missing fetcher=DirectKeyFetcher
time="2022-02-06T13:58:37.171866527Z" level=warning msg="Signature validation failed for event \"$nUsQL4zbQ1c4w6PBo_YZnY4PrIRAflvmMtjjuSZ6w0A\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"proprietary.lol\" with ID \"ed25519:bCfvh8\""
time="2022-02-06T13:58:37.171933868Z" level=warning msg="Signature validation failed for event \"$vQXj4wh5ikIq0d3o4YP-R9JSfxVLyYeN1wN4lRbnEME\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1625171310648"
time="2022-02-06T13:58:37.171965512Z" level=warning msg="Signature validation failed for event \"$tA36TpKlYOBPgd4iwgSxwdzDMzTQEN_9gndo_jsiGDc\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1624480683292"
time="2022-02-06T13:58:37.171994155Z" level=warning msg="Signature validation failed for event \"$opnx4MbHiyjpVXLLAUGqeS9pTesLku75vhyZv63aESU\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_iCck\" for \"050821.xyz\" not valid at 1639749673013"
time="2022-02-06T13:58:37.172021135Z" level=warning msg="Signature validation failed for event \"$LZUBb6InCd-LAX8XK2VSxhRAsKE6v_M8bipjP6sMEAQ\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"manuel.life\" with ID \"ed25519:0\""
time="2022-02-06T13:58:37.172046914Z" level=warning msg="Signature validation failed for event \"$Oqt_bOfPQ3jCPz-hN0DshyPESb5AV6qxtX2NuIMaNZE\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1625169545873"
time="2022-02-06T13:58:37.172073315Z" level=warning msg="Signature validation failed for event \"$xj97EFD4GLIu7KZXYxzyncGuAaVCk028c6hqS075ZjU\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_rRAI\" for \"satterfield.us\" not valid at 1611292731225"
time="2022-02-06T13:58:37.172100148Z" level=warning msg="Signature validation failed for event \"$In4o1J4STij69Ku25heRi7qp5qYUYIYDomUUKgRglvU\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: could not download key for \"dendrite.rom.dog\""
time="2022-02-06T13:58:37.172145098Z" level=warning msg="Signature validation failed for event \"$sdBf2fEEiJ2IqtlrMehO_2-d3tZduksGZuWrtILDUL4\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_WRob\" for \"hispagatos.org\" not valid at 1596997206194"
time="2022-02-06T13:58:37.172210785Z" level=warning msg="Signature validation failed for event \"$H2TVPESLexnOXFxL5u0kd9VxA01_J-lhtNwxcAQWCtM\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"proprietary.lol\" with ID \"ed25519:bCfvh8\""
time="2022-02-06T13:58:37.172276011Z" level=warning msg="Signature validation failed for event \"$afFI9Z9DlXdrM8B4n3tGrL0KCvaaP8iw0iNBcwxSiBM\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:NWbDrM\" for \"cripslock.undef.tools\" not valid at 1616293484998"
time="2022-02-06T13:58:37.172338901Z" level=warning msg="Signature validation failed for event \"$yf0_5aVue2aEPNxRiqpANEwv1NHNuPv2mCK3wBwpUVI\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1624480111032"
time="2022-02-06T13:58:37.172397334Z" level=warning msg="Signature validation failed for event \"$Y2YWwBkfsU4HI2L6fQxCdi3GxOCoyEGiVHjhmm8arOs\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:NWbDrM\" for \"cripslock.undef.tools\" not valid at 1616281905787"
time="2022-02-06T13:58:37.172460346Z" level=warning msg="Signature validation failed for event \"$zmaN5OQvckXrjh_VcQdUtJt6bUt5JJTKaudyU8_GAT4\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:NWbDrM\" for \"cripslock.undef.tools\" not valid at 1616293465447"
time="2022-02-06T13:58:37.172544108Z" level=warning msg="Signature validation failed for event \"$PJ4qVwBpYGo5z0-FVeXQpFtN9-FdeALuoz8W4lqk-co\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_pqvl\" for \"matrix.riedel.dev\" not valid at 1576114959176"
time="2022-02-06T13:58:37.172613311Z" level=warning msg="Signature validation failed for event \"$dpKzDoIlxEiSeatagYxiaNVTD8iw5z1yLl_8MB3S0vU\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"bcow.xyz\" with ID \"ed25519:2bpeHI\""
time="2022-02-06T13:58:37.172661426Z" level=warning msg="Signature validation failed for event \"$Qw1xH1YDUn_HW4UW72wKz2ifdQpTL1pGkwf-OH9VLUE\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"bcow.xyz\" with ID \"ed25519:2bpeHI\""
time="2022-02-06T13:58:37.172689578Z" level=warning msg="Signature validation failed for event \"$vLzEYeff1nwRFpSfzdU06bDV-7I5gQJO5XehfDrwiA8\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"yiff.social\" with ID \"ed25519:L1RXQh\""
time="2022-02-06T13:58:37.172745412Z" level=warning msg="Signature validation failed for event \"$5NVCP7Qpel04r7pQq_FGL-g3aWozYbvEt2DTxh1wqqw\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"bcow.xyz\" with ID \"ed25519:2bpeHI\""
time="2022-02-06T13:58:37.172790891Z" level=warning msg="Signature validation failed for event \"$ujuHayQfxcEF4KDOcX1skapZAX5crBq5c_oUrY1nLzs\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_ddyx\" for \"1bit.se\" not valid at 1607793481969"
time="2022-02-06T13:58:37.172835247Z" level=warning msg="Signature validation failed for event \"$L_Pxtj7qwKG-nBIA_ScRDqNh6k8Wgf1R0HWY_EtzAGk\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:0\" for \"nwca.xyz\" not valid at 1592406093989"
time="2022-02-06T13:58:37.172879463Z" level=warning msg="Signature validation failed for event \"$fk9Zum02DzZHetLONYUqJ25o3nAJ9S81bmsKd-H7Dko\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1624479888688"
time="2022-02-06T13:58:37.172923299Z" level=warning msg="Signature validation failed for event \"$hhSQL0cWn7_fweP05_Im6dJvjydbecQ5ZrzN5rhZx2c\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1625169326522"
time="2022-02-06T13:58:37.172974583Z" level=warning msg="Signature validation failed for event \"$coc3xMJT9gSPpP0HZ_sMVLCDWnyEr06ORaQrkg9n-aA\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"yiff.social\" with ID \"ed25519:L1RXQh\""
time="2022-02-06T13:58:37.173004263Z" level=warning msg="Signature validation failed for event \"$DU-1BZvFyxcTYdLdshN5Iiwrl0Bion_ymYPG5kzAvCU\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"proprietary.lol\" with ID \"ed25519:bCfvh8\""
time="2022-02-06T13:58:37.173042613Z" level=warning msg="Signature validation failed for event \"$0ES4-kHuqyWenIQN_NFe02mH31i1ohp9n7yQcBG1YKc\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"manuel.life\" with ID \"ed25519:0\""
time="2022-02-06T13:58:37.173079307Z" level=warning msg="Signature validation failed for event \"$UELQFbozuqvT1W1d612ubs6YxHfvhLMxjTocnbDeo-I\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_rRAI\" for \"satterfield.us\" not valid at 1611291871075"
time="2022-02-06T13:58:37.173116199Z" level=warning msg="Signature validation failed for event \"$4zICntTOmCIuHjCgcj_6yK2WpyneES6G-419cbb9Ngs\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_WRob\" for \"hispagatos.org\" not valid at 1576626668422"
time="2022-02-06T13:58:37.173153928Z" level=warning msg="Signature validation failed for event \"$yNaAliPolhDitOYu0arjawZbIe8Av-g1T1CMc1EJ_1o\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1625170070703"
time="2022-02-06T13:58:37.173206594Z" level=warning msg="Signature validation failed for event \"$cvjQu_m2cE1C7TtlSpEbpAJpx9c8avlwlq5SF_GPvOY\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:8KV0b9\" for \"thejoyo.com\" not valid at 1624480844485"
time="2022-02-06T13:58:37.173252463Z" level=warning msg="Signature validation failed for event \"$TWODd9VPdZOMn5zia2RfMwTVyeMNm5_KfpPTVAJ5zos\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:1orNdp\" for \"otsuka.haus\" not valid at 1636129968358"
time="2022-02-06T13:58:37.173299120Z" level=warning msg="Signature validation failed for event \"$Av-ZvCLQXtPDviPO6O_7LKFGERF3c_twlXwHxrX81t0\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:0\" for \"nwca.xyz\" not valid at 1592407546706"
time="2022-02-06T13:58:37.173356801Z" level=warning msg="Signature validation failed for event \"$UAe3OpyJWe3h6YC-wFRJOP6LF10Yj3mmbDAqvBosk3Y\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_iCck\" for \"050821.xyz\" not valid at 1639820924257"
time="2022-02-06T13:58:37.173403740Z" level=warning msg="Signature validation failed for event \"$fdt5MO-aYFTVG17zAwWBCglWUYnUagFYXfrgB_obxCw\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"bcow.xyz\" with ID \"ed25519:2bpeHI\""
time="2022-02-06T13:58:37.173438840Z" level=warning msg="Signature validation failed for event \"$ZEDC7EHXMgwzIbrieNi0w-S15VBfAoikf7M3uCrb7Jc\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_WRob\" for \"hispagatos.org\" not valid at 1578677304783"
time="2022-02-06T13:58:37.173482676Z" level=warning msg="Signature validation failed for event \"$zisn4mEm2_IqZ-3wIuUTsn7qpTN31yeI2do8NtSHt_I\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"proprietary.lol\" with ID \"ed25519:bCfvh8\""
time="2022-02-06T13:58:37.173529233Z" level=warning msg="Signature validation failed for event \"$7xFvGFT8AsNilDhpfLuV7_qIIHM5Rv84j4XwwjPSud8\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"manuel.life\" with ID \"ed25519:0\""
time="2022-02-06T13:58:37.173572330Z" level=warning msg="Signature validation failed for event \"$Zr_n3zOge7q3DfBYpjFKC9pUdWSSRpVIXz-FY_12LO4\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_QChh\" for \"quincystrange.com\" not valid at 1605559323478"
time="2022-02-06T13:58:37.173615262Z" level=warning msg="Signature validation failed for event \"$-3HbQ3RkqyG0QmN21EpE8rENhyuGV3w04IaQJZ_HA7U\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_ddyx\" for \"1bit.se\" not valid at 1610379720212"
time="2022-02-06T13:58:37.173680078Z" level=warning msg="Signature validation failed for event \"$TQ_uGyD8qRJmFl6sBv0OhNlV6o7qPz2nESAhpzPpJe4\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: could not download key for \"testuser-dendrite.duckdns.org\""
time="2022-02-06T13:58:37.173723516Z" level=warning msg="Signature validation failed for event \"$b70p5CMIUZRkutxmy-mX2YjgG34QwwzZDpFg9xqW6fE\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="Bad signature from \"toke.dk\" with ID \"ed25519:xPlL9W\""
time="2022-02-06T13:58:37.173759698Z" level=warning msg="Signature validation failed for event \"$1LCTj6oPYWbxCGBnCLf4w9IEh27CitNPOHiYWCOoLNk\"" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:525" error="gomatrixserverlib: key with ID \"ed25519:a_QChh\" for \"quincystrange.com\" not valid at 1607032247837"
time="2022-02-06T13:59:02.329573951Z" level=warning msg="Discarding 42 auth/state event(s) due to invalid signatures" func="github.com/matrix-org/gomatrixserverlib.(*RespState).Check" file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20220204110702-c559d2019275/federationtypes.go:549" context=missing
time="2022-02-06T13:59:13.809501594Z" level=warning msg="Auth event $s0aNapxPDZFPpV8jjKb6WylujsQX2feITeU2H6HldS8 rejected" func="github.com/matrix-org/dendrite/roomserver/internal/input.(*Inputer).fetchAuthEvents" file="github.com/matrix-org/dendrite/roomserver/internal/input/input_events.go:454" context=missing error="eventauth: sender \"@abuse:matrix.org\" is not allowed to send event. 50 < 100" event_id="$n5OLkHun_GvxmsNuwLEGrT7W3atCH4ElZMQ86P8N2Pw" kind=KindOutlier origin=matrix.org room_id="!yomrOFwgFXzmeMAbzX:matrix.org" type=m.room.member
time="2022-02-06T13:59:13.933285380Z" level=warning msg="Roomserver failed to process sync event" func="github.com/matrix-org/dendrite/roomserver/internal/input.(*Inputer).InputRoomEvents.func1" file="github.com/matrix-org/dendrite/roomserver/internal/input/input.go:239" error="missing auth event $coc3xMJT9gSPpP0HZ_sMVLCDWnyEr06ORaQrkg9n-aA" event_id="$n5OLkHun_GvxmsNuwLEGrT7W3atCH4ElZMQ86P8N2Pw" room_id="!yomrOFwgFXzmeMAbzX:matrix.org"
time="2022-02-06T13:59:13.933418379Z" level=warning msg="Failed to join room through server" func="github.com/matrix-org/dendrite/federationapi/internal.(*FederationInternalAPI).PerformJoin" file="github.com/matrix-org/dendrite/federationapi/internal/perform.go:101" error="roomserverAPI.SendEventWithState: InputRoomEventsResponse: missing auth event $coc3xMJT9gSPpP0HZ_sMVLCDWnyEr06ORaQrkg9n-aA" room_id="!yomrOFwgFXzmeMAbzX:matrix.org" server_name=matrix.org
time="2022-02-06T13:59:13.933487544Z" level=error msg="failed to join user \"@rbn:dendrite.collec.tf\" to room \"!yomrOFwgFXzmeMAbzX:matrix.org\" through 1 server(s): last error roomserverAPI.SendEventWithState: InputRoomEventsResponse: missing auth event $coc3xMJT9gSPpP0HZ_sMVLCDWnyEr06ORaQrkg9n-aA" func="github.com/matrix-org/dendrite/federationapi/internal.(*FederationInternalAPI).PerformJoin" file="github.com/matrix-org/dendrite/federationapi/internal/perform.go:129"
time="2022-02-06T13:59:13.933544706Z" level=error msg="Failed to join room" func="github.com/matrix-org/dendrite/roomserver/internal/perform.(*Joiner).PerformJoin" file="github.com/matrix-org/dendrite/roomserver/internal/perform/perform_join.go:62" error="5 : roomserverAPI.SendEventWithState: InputRoomEventsResponse: missing auth event $coc3xMJT9gSPpP0HZ_sMVLCDWnyEr06ORaQrkg9n-aA" room_id="#dendrite:matrix.org" servers="[]" user_id="@rbn:dendrite.collec.tf"
```

Start at roomserver/internal/perform/perform_join.go:59 and trace the federated join while comparing the reported failures in gomatrixserverlib federationtypes.go:515 and keyring.go:263-497. Reproduce the join against a medium-sized room and determine whether the join completes despite key-fetch and signature-validation errors; done means the affected rooms can be joined successfully.

Written by the indexing model from the issue text.