Bumping this...
I have the same issue:
I can use a save game created on Mac on my Windows PC, but if I load a game saved on PC, the game crashes on Mac.
This issue appears whether the save game is transferred by Steam/Larian cloud or by copy/paste from one computer to the other.
A game save perfectly working under Mac OS can be opened by PC, but once it has been saved on PC, it becomes useless under Mac OS.
The crash generates the usual lengthy and incomprehensible report that is sent to Apple.
Any solutions ?