I have a small update.
since my last answer my friend has completely deleted his phone and installed the new version LineageOS 17.1 Stable (Android 10).
He has deleted every Android app from his active sessions and installed the app completely new.
Unfortunately he still doesn’t receive any notifications. After that we tried every available mattermost app (mattermost classic & mattermost beta from the fdroid store as well as mattermost&mattermost beta apk from the google play store). but unfortunately he still doesn’t receive any notifications.
His older phone and his windows client, still get all notifications.
we have checked all notifications and app-specific settings, but it looks exactly the same as on his other phone. the test notification, which is issued by the app itself, also works as it should.
since the app is available in the f-droid store, i don’t think it has anything to do with the missing google play services. unfortunately i can’t find out what’s wrong.
is there anything else i can do?
Thank you very much.