I don't know if this will work for everyone but I just wanted to say that for me what I did to fix the serial key being in use for MP problem was to make a profile in Crysis 2 so you don't play on the default profile. Then you login to MP and enter your key first (before login details, at the bottom click "modify") and type your credentials (user & password) in afterwards. Check the box to remember your credentials, or if it's already checked just uncheck it and check it again.

This should get you in to the MP, and unfortunately you'll have to manually enter it every time to play MP (until they fix it, or if they fix it) but you only have to enter the key like in the box by itself when the window pops up, not the whole login screen thing like I said in the beginning of the post. THAT part is only done once. There you have it, working MP with the annoying case of having to re-enter the key every time until they fix it being saved.

I know this won't fix it for everyone but maybe it'll work for you like it did for me.

Well, they should fix it, since typing an string like that every time you want to play is... Well, garbage.

Well, they should fix it, since typing an string like that every time you want to play is... Well, garbage.

Agreed. But this temporarily remedies my inability to play at all, so I just thought I'd share.

Didn't work for me.

This is totally unacceptable. I mean seriously. I just want to play the F'ing game.

It is sort of lame when you can not use the MP they way you want to on launch.

At least the Singleplayer is nice and long. I am enjoying that.

I bought the game solely for multiplayer. This is the worst launch ive seen in FPS history.