I am not getting any errors with your saves, so the problem is likely in the configuration or the game itself.
Try verifying local files: in the Steam library, right click on Divinity Original Sin Enhanced Edition and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin Enhanced Edition and select Properties, then switch to the Updates tab and check the Steam Cloud section).
Next, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin Enhanced Edition' 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. With Steam cloud support enabled, Steam would just download the cloud copy of your existing profile.
After that, start D:OS EE, create a new profile and see if you can start a new game and save/load. If that works fine, exit and copy a couple saves from the renamed folder into the newly created profile's savegames folder. If that lets you load the saves without an error, move the rest of the saves over.
If you still get an error, delete the new My Documents D:OS EE folder and rename the original back again.
