after many hours of researching by myself I found a possible solution for logging in into gamesas.com and redeeming your codes here.
So listen up:
As you may already know the gamesas.com profile is linked in some way with your GameSpy account.
So... first make sure you know your Crysis 2 nickname, which you use for logging in to the game.
Ok, there you have your nickname.
Now login to your GameSpy account with your normal password you always used for GameSpy.
If you do so you have now the choice to choose one of your nicknames (if you have more than one) to login.
And this is exactly the point where the shiatz worked for me.
When you see the nicknames in GameSpy which you can use.. there are two. One normal and one in a brace. For example:
CrazyMufaFuka1 [retardedr0xx0r]
GameSpy nick | Crysis 2 nick
It worked for me this way: I used the GameSpy nickname (the one next to the brace) ... this would be CrazyMufaFuka1 in our example and my Crysis 2/gamesas.com password to login.
Yes, when you are logged in there it shows the GameSpy nick, BUT it is your right e-mail address attached to this nick, with which you registered @gamesas.com!
Tell me if it worked for you, too.
Good luck and many kill streaks!
Lord Abi