It appears that I've found the fix.
Follow these steps:
1. Enter the game
2. At the log in screen, click "modify" under the serial key entry.
3. Type in your serial with dashes, ALL LOWERCASE
4. THEN retype all user credentials (username and password)
5. Untick the check box next to password, labeled "remember details".
6. THEN re-check the box.
7. Log in.
*Note: If it tells you that your credentials are not correct, re-enter your password.

Let me know if this works. If not, I have a few more solutions. It's possible to get past this error with a little patience. It all has to do with the login screen. Your game isn't broken.

Bump

nope, not a thing. every time i restart the game the code becomes invalid again. thus, the game IS broken.

Try this. Enter your key. Retype your user credentials. Intentionally enter your password wrong. Enter. It'll give you a warning. Retype your password. Then uncheck "remember", recheck "remember" and log in. See if this does anything. Keep messing around in the log in screen. It will work eventually.

If this fails -- intentionally enter your serial code wrong by one letter or number. Attempt to enter. Change it back to the correct and try again. Eventually it should remember your key, as it did mine.

ive done everything youve suggested. im glad it worked for you but its not doing anything for me at all.

I was having the same problem. I solved it by doing the above and after logging in, joining a multiplayer game. It seems the game doesn't save your key until you have fully entered a match.

I was having the same problem. I solved it by doing the above and after logging in, joining a multiplayer game. It seems the game doesn't save your key until you have fully entered a match.

Were you able to fix it by doing what I suggested in the initial post, or what I suggested later in the thread?

thing is i cant join a match cuz its sayin serial code in use. its not just not saving the data, its letting me get all my limited edition stuff, and i can search for servers but then when i try to play it says "serial code is currently in use" or something like that. im not trying to be all like "EA YOU svck" blah blah blah, just sayin that what was suggested isnt workin for me. and i cant figure it out myself, im not a programer or someone who is good at figuring out stuff like this.

ps single player is pretty cool, at least because of all of this i was forced to play some single player campaign, even tho i wasnt even gonna play it at all, just wanted to jump straight into multiplayer... you know how thats going.

Just run the game in administrator. It fixed the problem for me. now if they can just fix the hard crashes. Seriously it's one thing after another with this game.

i did that too... administrator didnt do nuthin for me. man im so bummed out. should i like reinstall it?

Yeah this still isn't working for me, this seriously needs to get fixed. What an epic fail. I get the limited edition which gives me a headstart and I can't even play the game? Soooo retarded. This launch is so fail you'd think it was an MMO

So, I have tried everything suggested in this thread and no dice, also upon entering an invalid cd key I was still able to connect to the multi server browser, but alas the same **** about the "serial key is in use" wtf? I assume that it is an issue with the Crytek server not being able to identify serial keys. Or it may even be a Gamespy issue, after the getting dropped from servers issues with Gamespy about 7 months ago in Crysis Wars, I have little to no faith in anything powered by Gamespy. So after typing this crap I may go try to re-login with Gamespy or maybe launch from Gamespy Comrade I suspect this may be a reasonable fix seeing that I haven't done this yet I only launched from the executable. Then again I could be full of **** and nothing I just said will make a difference but wth I got nuthin better to do then try to get this fixed other than go play BFBC2. Whatever the outcome I will repost, given the fact that I have seen not even the slightest effort from Crytek to address this problem I am losing faith with them and if I lose that they will lose my money.

I did what you suggested in the initial post. Although I didn't bother retyping credentials and unticking and ticking boxes.

no dice launching from Comrade but I was able to connect to an empty server and wait till others joined but as for joining servers with other players I'm still getting the same old ****. Not very **** happy about this at all.

I did what you suggested in the initial post. Although I didn't bother retyping credentials and unticking and ticking boxes.

would you be so kind as to enlighten us on the outcome of your attempt or does this remain a mystery for the rest of us to ponder upon?

Yup, i got all the same problems you guys got. This is such a load of ****. Did Crytek even test this game? You'd think something like this would be easily avoidable.

On top of the serial code error, every time i try to enter the create a class screen with my limited edition content crysis 2 crashes. Fantastic. Anyone else have that problem?

i cant even get my limited edition content to show up on the website i enter my key and get a black screen