I have found some more.
This is apparently a known issue that is caused by changing the Monitor IP on an OpenVPN-Interface.
Here is the bug report: https://redmine.pfsense.org/issues/8142
And here the discussion linked in the report: https://forum.pfsense.org/index.php?topic=138608.msg764734#msg764734
The issue is still present in 2.4.3-RELEASE (amd64).
The only workaround I have found without resetting the system was to change the subnet of the Ubuntu OpenVPN-server to something different than x.x.x.0/24.
x.x.x.0/24 seems to be forever blocked by the non removable route.
If anyone has any updates in that regard, I would be highly interested, so please let me know!
Kind regards,
Holger