A guy i play with had this issue and turning off "SteamGaurd" got him back in the game.
I had this same problem......
TRY THIS thread for the answer.
""Go to steam library, right click on COD BO MP and click properties. Select the local files tab and then chose the verify integrity of game cache files option. I had this problem with another game and my game files did not match the files steam had and got that same error. The game was patched Friday so maybe the patching process did not work correctly on your machine?""
I'm telling you it's has to be an issue with the server host, gameservers, because it's a localized issue. If it was a game issue or a steam issue, it would be widespread. I haven't seen anyone from the US post with this problem. It seems to be confined to Europe. At least that's where I would start to look.