The last I heard of this error, it occured when someone was viewing the minimap (by pressing tab). Regardless, have you tried to re-install the game? An Assertion Failure is an illegal situation that the programmers knew of, and tried to account for. They should only really happen once every blue moon. If they happen alot, you know there's a problem with the files or your system, but because you mention you haven't changed anything with your system recently, it could be a file issue.