The g/f and I each have an XBox, same network. Both reporting open NAT both in XBox and BB.
We both start the game, doesn’t matter which one turns on XBox or BB first (which means not UPnP or port mapping issue).
Start the game, either she joins me or I join her, doesn’t matter. Start private match or public match, and one of us ends up timing out.
Quit and restart BB (from xbox menu) sometimes we can get 1 game in.
What I’ve found, The player that gets the timeout needs to restart their BB. Then join up again when it loads, start the match, the first player that didn’t get the timeout now gets the timeout. Second timeout player restarts BB, rejoins First timeout player, now a match can be played. If we’re lucky, we can play 2 in a row without doing this dance again.