I believe the last time that issue happened, Kalvan advised that they redownload the game completely, although I don't remember specifically why.
For your own sanity, you might want to try updating Direct X again, just in case. If that doesn't do it, and you still get the error message, and redownloading the patch (again) doesn't work, just skip right to redownloading the game, unless one of the other forum brainiacs has a better idea, that's all I've got