Okay Hektor i know this problem and i also know why this happens...
So its the big question did you bought the jk3 game on ebay?
IF you bought it on ebay it might be just a copy of the original cd so the one who sold it to you is playing the mp^^ so its like the same CD id try to get online at one time
You shld buy a new one wich is no copy^^
U might think yours is no copy BUT you dont know how exactly copys ca look like^^