It's quite much the same problem as explained in Battlelog, I want to see if I have better luck here than: http://battlelog.battlefield.com/bf3/forum/threadview/2832654625565972218/
Explaining it, I was playing with a friend on a Co-op mission of Battlefield 3, when the game crashed. We played a lot before that, but since, we can't play together, as every time we try Battlelog says that my friend is already playing co-op with me and doesn't appear on the list. Same happens to him. I can play co-op with public matchmaker or any other feature of battlefield 3.