
Ffffffuuuuuuuuuuu-
Could be the Steam Cloud, ever since people switched over to Steam 2010BETA it's been buggy. I've had the same error with TF2, but it said I didn't have a valid account to play. Try re-installing or re-logging in. I didn't try reinstalling TF2 but I read a thread on the Steam Forum with your same problem and that seemed to be the answer. I just waited until the next day and suddenly I could play again.