Try creating a new profile and then start a new game.
Most often this error has been the result of using an earlier version of the save editor, or a couple times an error copying a save from one computer to another, etc (which re-copying would generally fix). It indicates there was some change in the save, but not whether it may be serious or not. The fact that something carried over into the new game is concerning, though.
If it still happens in a new profile, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin' subfolder. If applicable, for a cleaner test you can disable Steam cloud support 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 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
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, create a new profile and see if you can quickly start a new game and save/load. If that gets rid of the error, you can restart with the characters that you want. If you have the GOG version of the game, you would need to copy the 'DLC' folder from the renamed folder to the newly created 'Divinity Original Sin' folder, so you would have the companion DLC and (if applicable) the Source Hunter DLC enabled when you next started a game.
If you just want to get rid of the error, you can load a save in the Save Game Editor
(Windows program, apparently can be run in Wine on Mac), extract the contents of the save, then in the save folder (in '..\Documents\Larian Studios\Divinity Original Sin\PlayerProfiles\<ProfileName>\Savegames') move or rename the .lsv file.