"PART" OF THIS SOLVED FOR ME, SEE BELOW, may help u, but idk
well, same here. Steam version
For the demo, and now live version, I could not use my gamespy credentials which worked fine for gamespy
Could not create an account in game with my email address because it said my email was in use
Could not reset whatever password is connected to the email, because it just takes me to this site where I am able to create the username I want, with my email
Could not use my forum credentials from this site to enter the game, and so I had to make a fake email/pw/username just to get into demo
Could not get in with full version unless I use the fake credentials I made for the demo that are different from the ones i'm posting on here with
Cannot play multiplayer, even with the fake credentials i created because it tells me my serial number is in use
Edit
SOLVED PART 1
So I hit the link on the bottom of the in game login screen which took me to the site here...account details.
I changed my password to something else, then changed it right back again to the one i had before, and restarted the game, and I guess it re synced or something, cause now it lets me login with my actual username and password from here "prevelance" It then asked my for the serial number again, which i entered by hand, including dashes, and i got in.... (pw & username are same on gamespy which I changed a few days ago fyi, but i dont think it matters cause i don't think they are connected, I think they used to be connected, but don't sync with gamespy anymore perhaps, which would explain why they are stored from Crysis 1 but wont sync if u change a pw on gamespy)
NOT SOLVED:When I try to play multiplayer, it still tells me my serial number is in use.

And I certainly dont wanna enter the LE serial in for the bonus stuff incase it gets lost and stuck "in use" separately. Not that i would know where to enter it anyway. lol
Edit
NEW PROBLEMI guess this is happening to people already too, it asks me for my sn everytime I enter the game now.
At least a courtesy comment on the forums crytek???? Ive never seen a game team be so invisible from their customers. Perhaps this section should be changed from "Feedback & Support", to just "Feedback" ?? Maybe?