I exited the game and reopened. When the log in screen appeared, I went to "modify" under the security code section and re-entered it using NO CAPS AND NO DASHES. I then logged in as normal. It worked: it found my redeem code and it allowed me to enter the multiplayer lobbies with no further hitches. Hope that helps

Are you using the SERIAL Key? NOT the Limited Edition DLC weapon scar and etc key? I tried both with dashes and no dashes and i'm still getting Serial IS in use. Pisses me off.