ok, looks like something is wrong with the backup, is this the only backup you have? what exactly you want to get our of this backup? (calendars, contacts …?)
it is not clear if something happened during the creation of this backup.
I would like to get to the bottom of the issue and fix it but that maybe impossible at the moment.
I need from nextcloud:
In between: i have two more backups one/two month before. They also don’t work. When I load other backups, it seems nothing really happened, because only root user login is possible.
So backup function in old syncloud version don’t work well … 
Ok, backup was made cross platform recently also if you have a backup from more than one major version of Nextcloud ago then it will also not work.
On the other hand the error does not look too bad, can you run this after restoring the bad backup:
ls -la /data/nextcloud
Run this and try to login again
sudo chown -R nextcloud:nextcloud /data/nextcloud
Boris thx! This was the mistake. Everything works now! I will test everything in next days - i hope it stays like this!
Big thx for support!
Glad you survived this long conversation 
I will make sure system does this automatically: backup should not break /data/[app] permissions · Issue #680 · syncloud/platform · GitHub
Could you create new backup now and restore it to see if that works?
I cannot find anything in the restore procedure that could break /data/nextcloud permissions.
I tried it and it works!