I fixed the problem.
The firewall died during the night from friday to saturday, so naturally I needed to build a new one on sunday.
After a clean reinstall, I again started having the same error
openvpn xxxxx IP packet with unknown IP version=15 seen
Endlessly filling the logs, and killing the SSD-s.
It seems the ntopng is the culprit.
After disabling ntopng, the errors stopped. And after enabling ntopng, the errors started again, even when there are no clients connected, and the errors start and stop at random intervals.
I am currently testing running ntopng but without OpenVPN interfaces selected.
For now it seems to be working as expected.
So it seems running ntopng with OpenVPN interfaces selected causes the OpenVPN server to have endless errors, even when everything else is working fine.
Now I am waiting for monday so we have some user traffic, but judging by the short test I am currently conducting, it should work.
Hope this helps someone with a similar problem.