So... i bought Crysis 2 a couple of days ago from Steam, downloaded it and got the CD Key from Steam as well. The game is working fine on the Singleplayer, but i cant play on multiplayer because i cant change my Serial Code... i click on "MODIFY" write it with dashes and click ok. it changes until i click "login", then when i try to join a match it says that my Serial code is invalid, then i look and it changes back to "****-****-****-****-0000". so you might be asking yourself "why does it change back to that?" well... that because i wrote it first... and now it just wont change to any other thing...

Anyway, forget that... I'm not sure of what worked exactly but i did some things, I completely uninstalled the game using Revouninstaller, whitch deletes all of the folders and all of the Registry Entries related to the software, then i used Ccleaner just to be sure that the regystry was 100% wiped out of any Crysis 2 entry. Installed it again and deleted the Crysis 2 folder in "C:\Users\yourname\Saved Games". Then i started the game and wrote my login, then clicked on "MODIFY" and entered my Steam CD Key. Tried to play on multiplayer and finally, now the game remembers my CD key and i can play on multiplayer without any problems.

I have this same problem.. I'm not uninstalling and reinstalling.. not worth it imo.. Hope they fix it soon though.. would like to play multiplayer a bit.

Try to delete the Crysis2 folder in "C:\Users\yourname\Saved Games" maybe that will do the trick, if not then reinstall...