OK. I think i managed to solve the problem. I guess the problem was, that with the update, the user databases were corrupted.
I renamed all the user directories in the ‘datadirectory’ to USER_old, then deleted the user and created a new user with the same name. afterwards i copied the userdirectory with rsync back
sudo -u http rsync -aP USER_old/ user
After i’ve done that to each user, i readded the files to each user with the following OCC command
sudo -u http php73 occ files:scan --all
This repopulates the databases with the files in each folder (see following thread Tutorial: How to migrate mass data to a new NextCloud server )
(Note: sudo -u http → http is the user Apache uses under Synology, you may have to use www-data or ngix)