For some reason the game was not successfully completing the save process.
The game saves files to a temporary folder before moving them to a folder without the .tmp extension. This is to prevent a working save from being damaged if there is a problem during the save process when overwriting an existing save.
Check the number of files in the .tmp saved game folders. Missing or blank/corrupt files might cause the game to crash when trying to load.
A new game should have 22 files, after going to either Verdistis or the Dark Forest it should have 27, after going to both it should have 32, after getting to the wastelands it should have 37, and after getting to the Black Ring dungeon it should have 42.
Here is a file list of what should be in a save after going to Verdistis and the Dark Forest:
data.000
dialogs.000
extfree.x0
extfree.x1
extfree.x2
info.000
inv.b0
inv.b1
inv.b2
inv.i0
inv.i1
inv.i2
items.000
mapflags.000
mapv.0
mapv.1
mapv.2
objects.x0
objects.x1
objects.x2
quest_log.000
quickinfo.000
shot.xxx
shroud.x0
shroud.x1
shroud.x2
story.000
story.001
telpstates.000
world.x0
world.x1
world.x2
Different file extensions are associated with different areas of the game;
.0 / .x0 = Rivertown (Aleroth, Ars Magicana, etc)
.1 / .x1 = Verdistis, Council Hall
.2 / .x2 = Dark Forest
.3 / .x3 = Black Ring dungeons
.4 / .x4 = Wastelands
Since it happened for one day and then was fine, the most likely cause of this problem would be something running in the background interfering with the game (possibly a program that didn't exit properly).
Are you shutting down all non-essential programs (especially anti-virus) before starting the game?
Another possibility could be that a file became corrupt in the folder the game uses as a cache (
..\Divine Divinity\Run\Dynamic). Though rare, since files are created / copied there from the save game folder when you load, and written / copied from that folder when you save a game, maybe an initial problem carried over into subsequent saves.
The files in that folder can be deleted; they will be re-created as required the next time you start Divinity.
One person had a crash in Windows 5 or 10 minutes after exiting the game, and on rebooting Windows decided there was a problem with the (zero byte) inv.b2 file in all the recent saves, and deleted them. Fortunately this file can easily be replaced.