My buddy and I were having the same issue and this fix both worked for us (he had steam key I had d2d key so it should not matter). Before you log in click the modify cd key button and enter your cd key in all lower case with no dashes. After that connect to a NON official crytek server (usually the USplay ones) once you connect in the lobby exit your game back to desktop. Re launch crysis 2 click the modify button before log in re-enter cd key lower case with no dashes launch crysis 2 click multi and attempt to connect to official servers. May not work for all but this worked for my buddy and I soooo best of luck

Doubt it will work. Why lower cases with no dashes? Kinda weird.

just give it a shot I have had a couple of people reply to me saying it worked for em. In the immortal word of grade school kids everywhere dont knock it till you try it.