Try browsing to the '..\Documents\Larian Studios' folder and renaming the 'Divinity Original Sin' subfolder.
This folder contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing.
After that, start D:OS and start a new game; save a few times, and see if you can over-write a save and load earlier saves. If that works, copy at least your latest save from the renamed folder into the newly created profile's savegames folder.
