My network is set to "Xbox Live" and my NAT rating reads "Open", but I could not join my friends' party or game last night. I had no trouble partying up with these folks the day I friended them, but ever since we haven't been able to get a game going (unless I invited them, and now that doesn't even work). I have no trouble with Halo 4 matchmaking, but I'm worried the issue will repeat itself with any new people I friend.
Strangely, in the roster section I'm listed as "Not Joinable" in red with a lock next to it. My friends logged off before I could check their statuses.
When I tried to join their party, I received an error message in the party box saying something along the lines of "There may be an issue with the network." When I tried to join their game I received a Halo 4 message telling me the connection "timed out". Neither friend seemed able to accept my party/game invites.
I would appreciate any assistance you can provide.