This started happening to me after I'd already played the game a day or two. It happened in the middle of play. When the server switched to the next map, I got the "Your CD Key is not authorized to play this game"... which, of course, it is, 'cause I was just doing it.
I already checked for typos. My key is clear to me, and correctly typed. I want to know what must be done to correct this problem so that I can play the game. I bought it, I put in your key, and it's not working on some multiplayer games. Single player is fine so far.