Thank you even more for the help then.
Both times I used the App to set up the key, both times right from the get-go (the second time I used the self-destruct pin to wipe it and checked the checksum before installing it).
I wiped it again, this time I started with the quick setup rather than the app, and am not upgrading the firmware (which I noticed is the same version anyway).
This has been going on on different computers, phones, ports and adapters, so if this time something happens as well I’ll have to look into asking for a replacement.
P.S.: I also accidentally entered a wrong password when I went on a computer that had the keymap set to a different language, but in those cases it was the symbols that were switched around; the problem I had with the passwords was that I was getting completely random stuff.
Edit: I tried backing up immediately after the quick set up and you were right, the error was different:
-----BEGIN ONLYKEY BACKUP-----No Backup Key - Follow instructions here https://docs.crp.to/usersguide.html#secure-encrypted-backup-anywhere
I entered the passphrase and now it works. I will test backing up and restoring as I go along, at least I won’t lose as much time restoring my stuff in there.