Constantly loses sessions and logging in occasionally fails

nextcloud constantly loses sessions after a certain time (differently). Both the sync client and the web client.
Furthermore, logging in only works after 2-3 attempts despite saved passwords (also affects all clients: web client, sync client, Opensync(DAV) and Androidapp).

Starting system: Debian 9 with PHP 7.0 and Nextcloud 15.
Bug exists since manual upgrade from Nextcloud 16 and the necessary distupgrade to Debian 10 with PHP 7.3. Nextcloud 16 was installed completely clean (only config file included). Upgrade to Nextcloud 17 and currently 18.0.1 did not bring any improvement.
All solutions mentioned in other forum posts have been tried. None of them had helped:

  • Replace the php.ini
  • Switching off the cron job
  • Switching off the caching
  • Uninstalling php7.0
  • Completely work through the installation guide of Nextcloud
  • Disabled all non-default apps

If the session is lost, the debug log simply says so:
Login failed: ‘xxx’ (Remote IP: ‘xxx’)

What could be the error here?

I have the same problem, but only with a single user. Did you find ny solution to you problem?