Okay, switching the database backend in #NextCloud
solves my issue
seems to be not as much tested as they do with .mariaDB.
I'm a litte bit unhappy about dropping Postgres. But the cool thing with NextCoud is that it is so easy to switch the database
php ./occ db:convert-type --all-apps mysql nextcloud 127.0.0.1 nextcloud
For a smooth run a had to raise max_allowed_packet in /etc/mysql/my.cnf:
max_allowed_packet = 32M