So I’m suffering from this issue and I seem to be the only one with this problem so far. It’s not my connection or my xbox and I know this because I’ve joined games with different profiles from this same xbox on the same network. I’ve tried everything within my power to try to fix this problem and I can’t get it to work. I even signed in at my girlfriends house and had the same problem.
I can play every other game online just fine, just not halo 4.
I go to join a game and it searches for players for a really long time and then it looks as if I might be able to join a game and at the bottom of the screen it says something about establishing a new host and then it says that everyone has left the party and it restarts the search.
I have sucessfully joined a single game in the course of 3 months and when a new player joined it kicked me from the game and well, it was back to the same old thing.
I’m hoping someone can tell me whats going on, whether it be a member of 343, a forum mod or any other person with some insight to this issue. I’d really like to know why I can’t play. 
-Loyal fan, and dedicated player