That's all? If you really want to screw yourself over:
1. Deinstall the game
2. Delete any backup you may have of savegames.
3. Download the game again
9/10 times, that solves this particular issue. You usually get it from damaged savegames or other residue. In the same 9/10 cases, this happens through mods. It could happen on its own some patches back, but is virtually unknown to happen in patch 6.
