Missing files by Raze

If there are missing files in the save you can not load, re-installing the game will not help. Replacing those files from the most recent working game might help, but try it on a copy of the latest saved game folder.

Incidentally, this may be a problem with windows. I am familiar with a file manager which sometimes does not delete files when told to, on some systems (when deleting a large number of files, it might miss 1 or 2 of them). It makes the call to windows to delete the files, windows puts this on its todo list, tells the program the files have been deleted, then does not get around to actually deleting them. Something similar may be happening when Divinity is moving files from the temporary save folder.

Otherwise, re-installing may help. Zip or otherwise backup your saved games folder to another location first, just in case. Then un-install, re-boot, shut down all non essential programs (especially anti-virus), re-install the game, perhaps start the game and see if the problem persists (usually not necessary, but would help identify problems cause by the patch), re-boot, shut down all non essential programs, install the patch, re-boot, shut down all non essential programs, start the game. This procedure is a bit over-cautious, but the best way to avoid install problems.

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If the game crashes mid-save, all of the required files might not have been copied / created in the folder. It might help to make a copy of the save game folder, then replace any missing files with those from your most recent working save.
This happened to me once; I lost a very nice set of leggings, but that was it. Windows was reporting the wrong amount of free disk space (FAT32 partition), so the game did not give a warning before starting to try to save.

The .tmp folder is meant to prevent the game from overwriting a working save with a non-working one if there is a problem during the save. If they re-named the original folder, then just saved directly under the save game name, I think they would avoid some of the problems with this procedure