Thank you for this new tip, I think we found the problem: ports 80 and 443 are not open. After I installed nmap (which was surprisingly not present in my Raspbian installation), the output of nmap localhost reads:

Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
53/tcp  open  domain
631/tcp open  ipp

I guess I did something wrong when following the tutorial (or the tutorial had some mistake, but I’d me more inclined to think the mistake was mine). I will try to clear this installation on docker and start all over again, then I will check nmap localhost again to see if it works fine then.

Thank you very much for your support. I still feel quite lost, but I finally found out why this is not working and I can repeat the steps and pay special attention… or look for a different method (someone here suggested using Nextcloud All-In-One).