I also had these problems since nc 15 (PHP 5.6 -> 7.2) and currently also with a new installation of nc 19.0.0 (PHP 7.3 - Ionos managed hosting)
None of the workarounds worked.
The following entry (php.ini in the nextcloud directory) offered a remedy
session.gc_probability = 0
This works well for me - a prerequisite of course is that the cron job runs periodically