Did you check out the editor?
Delete the contents of the '..\SteamApps\common\Divinity - Original Sin\Data\Mods' folder (or similarly for the GOG version), as well as any test mods in the My Documents folder '..\Documents\Larian Studios\Divinity Original Sin\Mods'.
That error can also be caused by a missing configuration file. Try starting the game directly from the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file by right clicking and running as administrator. Then change some sound or graphics settings, exit and restart. If that helps, you should be able to re-start the game through Steam again without the error.