For the Steam version, did you try verifying local files? In the Steam library, right click on Divinity Original Sin and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
For GOG maybe try a clean re-install (un-install, reboot, shut down all non-essential programs, make sure nothing is left in the install folder and re-install the game and patch).
Did you try lowering the game resolution and graphics settings (disabling post processing, etc), to see if that would make a difference?
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, any crash dumps, and your saved games. If file size is an issue, any saves not related to the problem can be deleted (since this is consistent, you'd only need to leave one, by the fire surface).