I looked into /etc/firewalld, I found:
- firewalld.conf, from today
- firewalld.conf.dpkg-old, from 29 January, perhaps when I last reinstalled
- firewalld.conf.old is from 2021
I compared filrewalld.conf and firewalld.conf.dpkg-old and noticed that DefaultZone=public in firewalld.conf was instead DefaultZone=external in firewalld.conf.dpg-old.
I changed DefaultZone to external in firewalld.conf and restarted firewalld.service (systemctl restart firewalld.service) and now, I can access the web interface again.
This allowed me to run diagnostics and see that everything is ok.
So perhaps my upgrade is ok, the only issue was the firewall? I don’t know whether I got the solution right.