pretty much spot on, simple explanation too.

a programming element is missing or otherwise invalid. Usually caught in testing/debugging though... if it were something wrong with released code, I would imagine we'd see a rush of the error on the forums by now.

Most likely, a .DAT file got corrupted during download or something. Hate to make someone go through a 8GB download/install process again... but it might be "easiest" way to do it. BlueGartr or someone might have a torrent up of the last round of .DAT's though. Haven't looked into that yet myself, but if you frequent their pages, should know where to find it (not posting it here, out of respect for SE's rules).