Hello saverio,
well, looks like that some update caused that problem. As it run before as it should, i think it is NOT a server/router prolem, but might be an update problem. I have faced this too.
here is what i did, and i do suggest it to you, but it is your decision.
The MSFS2020 needed to be reinstalled, and NO CHANGING to the suggested path for installation. That helped me.
I had purchased the BOX Premium Deluxe version, but needed to RE-INSTALL it again via the Microsoft store. Downladed the base version, installed it, made the Update to Premium version, and had to Downlaod this Monster Patch ( 70GB ) again. I did NOT change
the suggested path when re-installing the FS again. ( no trouble with the ONE-TIME -CODE )... i just ignored it.
I know, that it is a pain in the a**, but it helped me.
Believe me, i had tried a lot within my router ( FritzBox ) regarding settings Ports, passwords, adding some more ports...
Nothing helped.
only the new installing of the 2020.
I know , sounds like sad news, i keep my fingers cross that it will then work for you.
Good Luck
Best greetings
Robert
ps:
also a good idea is to CLOSE the XBOX-Live app, and RESTART it again, then run the MSFS2020 and hopefully it works.
if you can read and understand german language, check here..
https://www.flusi.info/forum/index.php?thread/7856-fs2020-kein-server/