Once this happened, none of my old saves worked at all. I poked around some, but I wasn't able to figure out where it's saving the config files.
I tried disabling my cloud sync, and even deleting all the remote files for the game, still no dice. Trying it from fresh with the cloud enabled/disabled doesn't seem to matter. I sent a save made right away in the prologue to a friend, and he can load it fine. If I try to load the same save ingame before exiting, it the map darkens and npcs disappear. Exiting to the menu and trying to load the save just loops me back to the menu.
Tonight I will see if I can load one of his saves. From what Rumpelstilskin said, the problem may not be the save files themselves, at least the problem I am experiencing. It sounds like there may be several. I was about 30 hours in to my original game before I abandoned it due to other bugs that had broken several characters, but I had never had any issues with the save games.