Yeah, I need to trouble shoot this as well, I've been getting a ton of logging in issues or just plain crapping out on me in game. It says "Unable to connect to game server. Please make sure you have a valid internet connection". I have a valid connection, constantly rebooting my xbox x and we even got a new router. I don't have any other problems signing into other games or apps.
This seems to happen alot when I switch characters. I can log into one, and not the other. Very strange.
As this discussion is a few months old, we've decided to close the thread. If you need further help please feel free to start a new thread in Xbox Support.