I found this in combination with the mailapp:
and there is a github issue: NC21, postgres, Doctrine\DBAL\Query\QueryException: More than 1000 expressions in a list are not allowed on Oracle. · Issue #4584 · nextcloud/mail · GitHub
If you use redis, there is less stuff handled by the database and the system gets faster. But that more in general, not sure if that changed anything for this particular problem.