Error: ERROR CODE 218 - Unable to decrypt chat message (transient): Error in reverse signature: Failed to verify/extract sig: Message wasn't signed in method

Experiencing the following issue on both desktop and mobile. I've submitted a bug report from both the desktop and mobile app.

I see this error message when I open up a specific group chat. This group was created a few months ago and sat idle, then started being used in the past few days. There are less than 10 members. The group chat appears to be completely unusable due to this bug

Desktop stack trace. (Version 5.1.1-20191211223249+15bbb94c23 on MacOS catalina 10.15.1)
```
Error: ERROR CODE 218 - Unable to decrypt chat message (transient): Error in reverse signature: Failed to verify/extract sig: Message wasn't signed in method chat.1.local.getThreadNonblock
at new l (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:478984)
at file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:479580
at d (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:479619)
at file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:1593034
at file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3569127
at e._call (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3140947)
at file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3141087
at s (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3140727)
at e._fulfill (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3141057)
at s (file:///Applications/Keybase.app/Contents/Resources/app/desktop/dist/main.bundle.js:1:3140258)
```

Mobile stack trace (version 5.1.0-20191210210019+5d92cf7e30 on ios 13.3):
```
ERROR CODE 218 - Unable to decrypt chat message (transient): Error in reverse signature: Failed to verify/extract sig: Message wasn't signed in method chat.1.local.getThreadNonblock

n@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2009:740
h@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2009:1286
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2009:172
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:3767:4207
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2040:582
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1029
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1173
c@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:808
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1139
h@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:349
handler@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:3771:2749
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:3771:991
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2024:3115
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2024:3053
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1029
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1173
c@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:808
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:1139
h@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2015:349
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2024:1843
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2024:1675
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2024:1479
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2025:1551
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:2025:1855
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:3770:474
value@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:49:1268
value@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:26:3504
/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:26:754
value@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:26:2710
value@/private/var/containers/Bundle/Application/99F32A03-5B12-4F36-B4BF-54A409476C7A/Keybase.app/main.jsbundle:26:726
value@[native code]
```

Start with the chat.1.local.getThreadNonblock entry point named in the desktop and mobile traces, then compare the referenced main.bundle.js and main.jsbundle locations with the client sources. Reproduce the error by opening the affected group chat and determine what is required for its messages to decrypt and display normally.

Written by the indexing model from the issue text.