This happened to me on two occasions, once for seemingly no reason when I never used to use overrides (many years ago), and the second was when I installed a cave override that gave better roof textures, so ever since I have stopped using the cave override but carried on using others, I never reccomend the cave override any more because of that, it might have got updated and fixed but it's much harder to check that now.
When it was nothing to do with overrides (before I started using them) I just re installed the game and it fixed the problem. So I would recommend saving your overrides/haks and tlk files and do a fresh install, if it still happens after the fresh install re download all the ravenloft haks and overrides. Definitely save all the haks and overrides first thought because they are harder to find now without the good Neverwinter Vault.
It might also be worth using the system tools and checking your hard drive for errors, doing a defrag, and of course updating and doing a deep scan with your anti viral software, like a good spring clean.