Error code 526 is an error adding a file.
If this happens again, maybe try setting the game to run as administrator (right click the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file, select Properties, switch to the Compatibility tab and check that option... should be set by default for the GOG version), and shut down all non-essential programs (especially anti-virus) before starting the game.
