This just happened a few hours ago and i havn't been able to connect since, it's really annoying and i have no idea why.. I am playing multiplayer/versus fine on all games ALONE, but i cannot join his party/lobby/session. Thanks
Run a test of your Xbox Live connection. This sounds like an NAT issue, let us know if the test gives you a NAT status of Moderate or Strict.
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If your NAT comes back as Strict or Moderate, then look here for help to change it:
And post back your details in full (the bits that are in bold on that post) so that people may help
If you are wireless can you also state whether you are using the white or black external adapter or the Xbox 360S Internal adapter
I assume you have no problems joining other peoples games/parties? When you run MW3 or COD does your NAT show as open in the lobby?
your nat type can change from open to moderate or strict it is very annoying mine dose it all the time but open is the best nat type to have.
Try to reset your rouder