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Running Win10 64-bit, with GTX970 updated to latest drivers. Got a similar problem here. No CTD, but defaults to 800x600 whenever I run 1920x1080 in full screen.

Running windowed, switch to 1920x1080, restart then set to full screen doesn't work. Resets to 800x600 immediately.

Manually changing to .ini to 1920 x 1080 also didn't work, it simply runs at 800x600.

This is forcing me to use a lower resolution (I settled for 1764 x 992, not sure why this presented itself as an option). Works OK, but still bugs me why I can't run at 1920x1080.
I'm Running Win 10 latest build Resetting to fullscreen and 1080p. the exiting game and the setting to fullscreen worked for me. as stated above. after running a bunch of changes I repared & verified files first through GOG galaxy!
caranthir1976: Deactivate fullscreen, change everything you want incl. resolution and save. Close and restart the game and it will start in window mode. Open settings and change it back to fullscreen. Close the game, start it again and it will work.

This is the only way to fix this problem with W10 creators update. CU has some problems with older games and fullscreen.

If you want to play Drakensang River of Time, you have to use window mode and borderless gaming. The same with Dungeon Siege 2.

compatible mode and/or running as admin does not work.
Thanks man, you saved my nerves :)
This advice should get to GOG FAQ