So here is a better fix for you to try if tried out all of the "other" suggestions and they did not work for you. Play the game MW3 for a few matches online, then eject the disk. Pop in your Black Ops 2 disk, get the clan leader to send you a fresh invite, go into Elite and join. That simple match playing should help fix the Joining issue as well as the leaving issue. I have tried this out with all 5 of the clan members that I/they had the errors joining and it worked for all of them. I hope that this helps the rest of you out as well. I have to give Death a shout out for this one, as it was a test on his part that worked for him to give me the idea to test this out with my joins. So give him a thanks as well.