Now it’s up and running. I went through the second suggestion again:
reinstall from scratch
It went faster now.
Changed password of openhabian and changed IP address (yes - I use fixed IPs for my servers and printers)
Mosquitto Broker still had to be installed, but that can be done quickly via oh-config.
Now it runs with bullseye.
Many, many thanks for your support.
Now the next challenge: Update to OH4.
Do you have any best-practice suggestions?