if the file isn’t read only (options.txt in settings folder)
game is reinstalled and repair tool used
AV isn’t blocking the folder/has a manual whitelist exclusion for both documents grim dawn folder and game install folder/exe
game/steam is run as admin
only things left i know of is
make a backup of the mygames/grim dawn folder somewhere, then wipe the entire grim dawn folder itself and let game try fresh
or, it’s Onedrive and you didn’t manage to fully separate it
suggest following Powbam’s instructions here carefully and possibly give a look to his final comment at the bottom of the thread
(i know you said you checked it, but it has sneakily been the culprit before despite initially unlinking)