NOTE: I know this is an old question, but I wanted to try to help any future Googlers who are running into this issue.
Based on this thread from the Steam Community forums, it appears that this error was due to an issue on Steam's side.
If you're still having this problem, (and you're sure it isn't caused by an add-on) try reverifying your local content (as shown here: Is there a way to manually update a game?Is there a way to manually update a game?).