Some new information:
There seems to be something wrong with the user account. If I create a new one, the app works fine. I already tried a few things:
- adding user to admin group and back
- modifying password
- logging in via app QR code or password
How can I check my user or “refresh” it?
SOLVED:
There was an old share for my federated cloud ID from the Nextcloud of a friend. It wasn´t visible in his Nextcloud anymore, so he removed it from the database table oc_share and I had to remove it from my database table oc_share_external. Now the error 500 is gone!
So, the iOS app has a bug, with handling unreachable shares.
Hope this is of help to someone!