Thanks for your kind support.
Just a few last comments.
BluishHumility asked me to upgrade my system and resolve the issues with my nfs connections. That is all of minor importance.
To get rid of those settings, I have tried to test the VPN with a new and clean installation (accidentally on another computer). That did not change anything.
I read the " Openswan L2TP/IPsec VPN client setup - ArchWiki".
It seems to be outdated and does not work for me. BTW, two parallel approaches are described. The first one (networkmanager) did not work for me.
Trying to set up a container is not worthwhile and is a cumbersome method for me (wasted another 2-3 h).
Research on the Internet indicates that this has been a problem for ages.
Sorry, I have given up or decided to spend my time more wisely.