According to the logs:
EXCEPTION java.io.IOException: stream was reset: REFUSED_STREAM
this is a duplicate of nginx HTTP/2 incompatible with okhttp. Please follow up there.
It would be nice to change the URL of the server as my server has a new IP address which I cannot update in the app.
Changing the IP in the database only made the app crash for some reason.
I hope this feature can be added or explained how one can do this.
Duplicate of https://forums.bitfire.at/post/6487, please follow up there