Error at OIDC initialization - restart in loop

### Description

I've configured my OIDC server ([Authentik](https://docs.goauthentik.io/integrations/services/matrix-synapse/)) for Synapse using the official [guide](https://matrix-org.github.io/synapse/latest/openid.html#authentik).

My `homeserver.yml` is populated as this (the URL of the matrix server is `matrix.tld`):

```yml
oidc_providers:
- idp_id: authentik
idp_name: authentik
# discover: true
skip_verification: true
issuer: "https://authentik.tld/application/o/matrix/"
client_id:
client_secret:
authorization_endpoint: "https://authentik.tld/application/o/matrix/"
token_endpoint: "https://authentik.tld/application/o/token/"
userinfo_endpoint: "https://authentik.tld/application/o/userinfo/"
jwks_uri: "https://authentik.tld/application/o/matrix/jwks/"
scopes:
- "openid"
- "profile"
- "email"
user_mapping_provider:
config:
localpart_template: "{{ user.preferred_username }}"
display_name_template: "{{ user.name|capitalize }}"
```

However, at the `docker compose up`, the synapse server restarts **in loop** because it can't initialize the provider. It seems it reaches a timeout, from the logs, *although it doesn't even wait half a second and directly restarts*.

Here are the logs:
```log
matrix-server exited with code 0
matrix-server | Starting synapse with args -m synapse.app.homeserver --config-path /data/homeserver.yaml
matrix-server | Error during startup:
matrix-server | Traceback (most recent call last):
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 148, in load_metadata
matrix-server | await p.load_metadata()
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 610, in load_metadata
matrix-server | return await self._provider_metadata.get()
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 143, in get
matrix-server | return await self._cachedcall.get()
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 113, in get
matrix-server | self._result.raiseException()
matrix-server | File "/usr/local/lib/python3.11/site-packages/twisted/python/failure.py", line 505, in raiseException
matrix-server | raise self.value.with_traceback(self.tb)
matrix-server | File "/usr/local/lib/python3.11/site-packages/twisted/internet/defer.py", line 1999, in _inlineCallbacks
matrix-server | result = context.run(
matrix-server | ^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/twisted/python/failure.py", line 519, in throwExceptionIntoGenerator
matrix-server | return g.throw(self.value.with_traceback(self.tb))
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 133, in _wrapper
matrix-server | return await f()
matrix-server | ^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 622, in _load_metadata
matrix-server | metadata_response = await self._http_client.get_json(url)
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 571, in get_json
matrix-server | body = await self.get_raw(uri, args, headers=actual_headers)
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 656, in get_raw
matrix-server | response = await self.request("GET", uri, headers=Headers(actual_headers))
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 423, in request
matrix-server | response = await make_deferred_yieldable(request_deferred)
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/twisted/internet/defer.py", line 1078, in _runCallbacks
matrix-server | current.result = callback( # type: ignore[misc]
matrix-server | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 964, in _timeout_to_request_timed_out_error
matrix-server | raise RequestTimedOutError("Timeout waiting for response from remote server")
matrix-server | synapse.http.RequestTimedOutError: 504: Timeout waiting for response from remote server
matrix-server |
matrix-server | The above exception was the direct cause of the following exception:
matrix-server |
matrix-server | Traceback (most recent call last):
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/app/_base.py", line 259, in wrapper
matrix-server | await cb(*args, **kwargs)
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/app/homeserver.py", line 368, in start
matrix-server | await oidc.load_metadata()
matrix-server | File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 152, in load_metadata
matrix-server | raise Exception(
matrix-server | Exception: Error while initialising OIDC provider 'oidc-authentik'
```

But I can browse to the Element homepage, and then when clicking on "Sign in", I get these log lines:

```log
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /i18n/languages.241c278.json HTTP/1.1" 200 904 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /_matrix/client/versions HTTP/1.1" 404 555 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 2024/08/30 13:08:58 [error] 29#29: *6 open() "/usr/share/nginx/html/_matrix/client/versions" failed (2: No such file or directory), client: 172.18.0.5, server: localhost, request: "GET /_matrix/client/versions HTTP/1.1", host: "matrix.tld"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /img/element-icons/i.cf33278.svg HTTP/1.1" 200 516 "https://matrix.tld/bundles/e9035f460125f26b7b0e/theme-light.css" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /_matrix/client/v3/login HTTP/1.1" 404 555 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 2024/08/30 13:08:58 [error] 29#29: *6 open() "/usr/share/nginx/html/_matrix/client/v3/login" failed (2: No such file or directory), client: 172.18.0.5, server: localhost, request: "GET /_matrix/client/v3/login HTTP/1.1", host: "matrix.tld"
```

I have this page:


I don't know why is there a timeout. My OIDC server is using a self-signed certificate, but I've patched the Dockerfile to add the CA to the trusted store.

How could I further debug this ?

Thanks

### Steps to reproduce

- Setup Authentik as SSO
- Configure Synapse and Matrix with the official guides
- `docker compose up`
- witness the error when clicking on "Sign in"

### Homeserver

self-hosted, not federated

### Synapse Version

1.113.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL

### Workers

Single process

### Platform

Dockerized with docker compose.

### Configuration

_No response_

### Relevant log output

```shell
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /i18n/languages.241c278.json HTTP/1.1" 200 904 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /_matrix/client/versions HTTP/1.1" 404 555 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 2024/08/30 13:08:58 [error] 29#29: *6 open() "/usr/share/nginx/html/_matrix/client/versions" failed (2: No such file or directory), client: 172.18.0.5, server: localhost, request: "GET /_matrix/client/versions HTTP/1.1", host: "matrix.tld"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /img/element-icons/i.cf33278.svg HTTP/1.1" 200 516 "https://matrix.tld/bundles/e9035f460125f26b7b0e/theme-light.css" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 172.18.0.5 - - [30/Aug/2024:13:08:58 +0000] "GET /_matrix/client/v3/login HTTP/1.1" 404 555 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36" "172.18.0.1"
matrix-element | 2024/08/30 13:08:58 [error] 29#29: *6 open() "/usr/share/nginx/html/_matrix/client/v3/login" failed (2: No such file or directory), client: 172.18.0.5, server: localhost, request: "GET /_matrix/client/v3/login HTTP/1.1", host: "matrix.tld"
```

### Anything else that would be useful to know?

_No response_