I was able to restore Threema from a backup; but I don’t recall if the backup worked as expected
Reason: I opened up a new account on the UT device pretty soon, since I wanted to continue to use the instance installed on my other Android fairphone. You can’t use the same instance on different devices; so I did a complete threema reinstall, and then opened a “group chat” which includes both Threema instances (UT and Android f airphone ) and a “third” person to “sync” the chats, or at least the new entries with the other person.
You have to activate “polling” in the threema settings, since the push-notifications don’t work without google play store etc on Anbox, which doesn’t work, as far as I know. MicroG isn’t an option either, since you can’t “root” Anbox.
It is hidden in “About Threema”, -> “Error handling” or “Bugs” (don’t know for sure, in german it is “Fehlerbehebung”, it’s the last entry in “About Threema”) and there “Workarounds” -> “Polling”
Threema on Anbox is not very comfortable to use, it is reacting rather slow, you can’t use the camera (so you can’t read QR codes to verify users), or the microphone, (no calls possible!), or play videos someone send, so be warned… It crashes everytime you try to open “help”, very convenient indeed