Whenever I try to play Crysis 2 multiplayer it tells me that my serial key is already in use.
I have the Steam version of the game. Today was the first day I tried to play multiplayer.
Any suggestions on how to solve this?

Update: Inputing the serial code all lowercase fixed it.

I get the same error message. Tryed to input the serial code in lowercases as you suggested, but it didn't work for me... I'm F*CKING PISSED &%?&?%*?%