Slight change of topic, but I wanted to share a fantastic solution to a problem that has plagued me (and others) for the longest time. I enjoy a good game of multiplayer Halo 2 as much as the next guy, but every once in a while I’d get this error as I tried to join a friend’s party:
You aren’t able to join this party because we’re having network problems. Sorry about that.
Not only was it an exceedingly unhelpful error message, but it would occur at random, and even though there was a MS Knowledge Base article, there was no known fix, so you were SOL. Today after hitting the issue and googling it for what seemed like the 100th time, I found a site listing the following fix:
Turn off UPnP on your router.
It’s as simple as that. On my Linksys WRT54G router the setting is located at the bottom of the Administration panel. After my buddy and I both changed the setting, the problem disappeared, and we were able to play online and get or butts whupped. No guarantees that this works 100% of the time, but you’ve got nothing to lose. 
-JL
ok, what if you dont have a router but just havea straight internet connection to your computer. if the other person turnes it off does it work?
this kids solution worked PERFECTLY
instantly i could join my friend’s party
@Halo 2 fan:
Not sure about your situation, did it work when you tried?
this totally worked!!! we had 4 people trying to join each other…. frustration brought us to here… with in 5 minutes we were all in the same party. Thanks a million John!
what if you have the wireless adapter for the 360, and you’re stealing internet from your neighbor. how would u fix this.???? cuz thats what im doin. haha