Minecraft - LAN Games suddenly broke (Your friends are not playing Minecraft right now)

I have been playing Minecraft on LAN together with my kids for quite a while without any issues.

Game is hosted on PC, joined by PC and by Xbox.

A couple of weeks ago this stopped working. All of us were playing together in the same room, when the Xbox player complained about lag. We rebooted the XBox and since then he receives the message:

"Your friends are not playing Minecraft right now" - his friends are marked as online in brackets, but the world is not visible or joinable.

After *extensive* hours of troubleshooting, I can only conclude that this is a problem with Xbox.

In no particular order:

1. Not a problem with the game world or hosting. We are perfectly able to join the world from PC. I also joined without problems from an Android Phone.

2. Versions are the same. I checked for updates and applied those where needed.

3. I reinstalled Minecraft on Xbox

4. As mentioned, it worked and 5 minutes later it didn't. No changes where made to any configuration setup in the meanwhile.

5. At this point I do not have Xbox Live Gold, but again, this should not be required and it worked before without it.

6. Hard rebooted the Xbox. Also checked for XBox updates. I have my Xbox configured as always on to apply updates.

7. Updated the controller software to ensure multiplayer is ok. Also tried a different controller.

8. Even though no changes were done, I extensively tested and tried different configurations of my network setup. NAT, P2P, wireless, wired, the works. Everything connects, but the Xbox.

9. Account settings are fine. Again - this worked before - there should be no issues here. Just to eliminate it as a root cause, I removed the my kids settings and reapplied them. I had my son log in to another PC and his account connects perfectly fine
from there. My adult account on XBox faces the same - I cannot connect to the game from Xbox.

10. I've checked this for around 3 weekends now, just in case this would get resolved in the background. Nothing.

As you can tell everything points to this being an Xbox specific problem. I'm pretty sure that I tried everything that can be done from my side. Did you (Microsoft) apply any changes/updates to cause this issue?

I'm looking forward for a quick response/fix.

:)