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This has an issue in https://github.com/hjiangsu/thunder/issues/205
A 5XX error indicates the error is not on the client (so this app) side, but an issue with the server. So there is really nothing you or the app dev can do, the lemmy instance admins need to fix it.
Worked fine!
https://lemmy.ml/post/1392824 https://lemmy.ml/comment/1032441
TL;DR: Android build is going to happen, waiting on google.