MagicDNS not working

I have a RPi, with Home Assistant and Tailscale, connected to internet under a CGNAT. I was used to access remotely to the RPi using the awesome MagicDNS Tailscale service: I was able to ping the RPi and to access to its GUI simply using the hostname: .. Some weeks ago it stopped working. I did no modifications. Ping and Web GUI do not work from outside and the problem seems to be the DNS service, indeed the hostname cannot be resolved anymore (trying from different networks and devices, I always get an “unknown host” response). If I skip using the MagicDNS service and I use the machine’s IP address, everything works (ping, web gui, etc.).
I tried to re-install, re-configure, re-start, the Tailscale client on the RPi, the Tailscale client on my laptop and also to start/stop the MagicDNS service and to change the tailnet-name, without any success.
Any glue what could be the problem?