Make sure your firewall isn't blocking access for the game (EoCApp.exe and SupportTool.exe both need access).
See
Killer Networks NIC and can't load the game? Fix!, if applicable.
Try starting the game directly from the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file. Perhaps right click and run as administrator (or right click, select properties and switch to the Compatibility tab if that doesn't help).
Are you shutting down all non-essential programs (especially anti-virus) before starting the game? (though, I don't think anti-virus programs have caused problems starting a new game, just loading the game the first place)
Try browsing to your '..\Documents\Larian Studios' folder and deleting or renaming the 'Divinity Original Sin' subfolder.
This folder contains the saved games (or, where they would be if you were able to start a game and save), configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing.
If that doesn't help, hopefully the update planned for this Thursday or Friday will.
If you have not done so already, please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool.
I've been playing this series since the first game was released, and I'd love to actually be able to play this one.
