Just to be really upfront about this, did you do anything to the save files themselves or while in game through the console commands?
If you did, then there's likely nothing that can be done and that's the risk you take when you do that. If not, then it's possibly a bug their team can actually look into.
Have you tried turning down all of the graphics settings for the game to see if your system just can't handle loading everything in your old save world?