After
sudo apt remove freedombox ; sudo apt autoremove
the problem went away. So the problem seems to be caused by freedombox.
After reinstalling freedombox the problem came back.
sudo systemctl stop firewalld
solved the problem. So it should be some update to the firewall rules that caused the problem. The problem appeared after an update of freedombox. I guess my remote server is lost forever (or till covid travel restrictions are down) since I can’t disable the firewall there.