Hrmm me and my buddy were having the same issue and this fix both worked for us. 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 empty ones) once you connect in the lobby exit your game back to desktop. Re launch crysis 2 click the modify button before login re-enter cd key lower case with no dashes launch 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

This (finally) worked for me.
For anybody else who has this problem, I would recommend going through all the fixes one by one, and testing every step of the way, if your problem is resolved before you get to this one, then be happy

so, now that that is fixed, my ISP just has to deliver the connectivity they are supposed to, lol^^

So, I disabled read only on my entire User folder. Probably not a smart idea. Oh well.

3 minutes to disable read only on every game I've installed. <3 quad core.

Will edit with other user-end fixes in a few minutes.

Still can't get in. Come on Treyarch/Activision....er.... I mean EA...

had the same prob. make sure you put you serial number in in the xxxx-xxxx-xxxx-xxxx format, not xxxxxxxxxxxxxx. this is the only thing i did different and it solve both problems. hope it helps

add me to this list fml!

The special content code works only if you activitate it on crytek website to get it noticed in the system. HOWEVER when inputting keys and trying to JOIN multiplayer servers you get a "serial code is in use." Error. Doesn't work for me last night. I spent 1 and 1/2 hr trying to fix it. I even uncheck read me only in save file and it still marked read only for some weird reason when i quit the game to check the folder. I even created a new folder and i still get the same error. I have no idea what to do and i really really wanna play the multiplayer soon.

I was getting the same issue when I tried to join a multiplayer game it was saying " Serial number already in use"... I tried making a new profile name, entering the key in lower case with no dashes, and so on..none of that worked.. So I had the idea to uninstall the Crysis 2 demo, and once I did that, I logged back into the game, and entered the CD Key exactly how they say to do it(Caps, and dashes), and now the multiplayer works fine.

Read my other posts. I can get in online now but sometimes you get disconnected. I think it's more of a majority of server problems. Give them time they will fix it. I suggest keep bugging and trying. I got in however. At least today. Wish i had more time to play instead of like maybe 10 mins then boom you get disconnected from the host lol. svcks.

After you get into the multiplayer screen, at the bottom left does it say your NAT is strict? If so that could be the problem.