Thank you for the hint, Michael! It seems to be the bug described here: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/946
Also our IT-department was able to provide a workaround. We added a script in /etc/ppp/ip-up.d called 0001changeroutes. The content is #! /bin/sh -e ip a del $PPP_LOCAL peer <public ip> dev ppp0 After making this script executable I was able to connect to the VPN again. Best Marcel