This should be fixed in the next update (no ETA yet, but it is currently being tested).
In the current version of the game (3.0.165.9), during the save process the original and temporary files can (rarely) not be removed properly when overwriting a save (leaving a backup of the lsv file, uncompressed files and a 'LevelCache' folder), which causes the problem the next time the game tries to overwrite that save.
Deleting the save (possibly after loading it and making a new manual save, if it wasn't going to be overwritten soon anyway) should help.
You can also check the relevant save folders in Windows Explorer, and for any affected, either delete/move the save (if it would be overwritten soon) or just the extra files and folder. Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
Normally each save folder contains an lsv and png file, named the same as the folder they are in (the png screenshot file being optional).
