Hello. I decided i wanted to play multiplayer and when i try to log on and join a game it says serial code already in use. help?

Hrmm me and my buddy were having the same issue and this fix both worked for us. Before you log in click the modify cd key button and enter your cd key in all lower case with no dashes. After that connect to a NON official crytek server (usually the empty ones) once you connect in the lobby exit your game back to desktop. Re launch crysis 2 click the modify button before login re-enter cd key lower case with no dashes launch multi and attempt to connect to official servers. May not work for all but this worked for my buddy and I soooo best of luck

i dont know how but that worked for me. but i hope i dont have to do that everytime.

Shouldnt have to enter your information again I havent had to do it since I did that lengthy fix... also I have no idea how it worked either haha