These worked for me.. not sure they will for everyone.

I tried multiple times to get my prepaid code to enter and it never would work.
I enabled third party cookies which I had disabled and voila it worked.

I had to enter it with the dashes for it to successfully accept my promo code. I haven't looked at how to retrieve this stuff but it said it went through successfully.


I ran my Crysis 2 client as administrator (not sure if this mattered but just in case)

I couldn't play Multiplayer because it kept saying my serial code was in use as it has for many of you.
The cdkey also wouldn't save regardless of what I did.


Then I put my cdkey in withOUT the dashes (even though it instructs to put the dashes in)
It saved my cdkey WITH dashes and I can now join Multiplayer games.

MY pre order code works of course but i'm SICK of the damn dreaded "SERIAL CODE already in use" When i'm trying to join the server? What the fricking hell man? I did it without dashes and it didn't work.. EVEN with DASHES caps too. Cleaned my Cookies. I'm stumped. Did someone started pirating keys and used our legit copies or is it a simple game error? This is raging me! Single Player is amazing and i didn't wanna finish it today wanted to save myself more surprises but i wanna play MP!

It said same thing for me when trying to join a server.
I just entered my code in before logging in.. in lowercase no dashes and it now saves my serial code and allows me to join servers.

Only thing I did different besides that is enable third party cookies but as far as playing the game I don't see what that has to do with it..