i just had the same, but i was using invalid one indeed, since my legit code was in use, now i had to enter my legit code and it works. Goodbye pirates!!!
Yes, I had the same issue today. It seems that somehow my key was incorrect (even after long time playing with that one - maybe it changed spontaneously to an incorrect one some time and I did not noticed), I just retyped it and now everything is fine.
Great. What good is launching a patch to fix now prevents multplayer I play? The game now says that my serial is invalid! Congratulations Crytek. So out of curiosity, you know what are they doing?