Game won’t load on Mac

Shut down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the Documents folder. 

Try verifying local files: 
Terminal 
cd ​/Applications 
codesign -vvv Divinity\ -\ Original\ Sin.app 

For the first command, if the game is installed to a different folder it may be easier to browse to the install folder in Finder, then drag and drop the location into an open Terminal window. The second command takes a while, and will output some lines starting with '--prepared' or '--validated'. If everything is fine, it will end with: 
Divinity - Original Sin.app: valid on disk 
Divinity - Original Sin.app: satisfies its Designated Requirement 

Hold the Cmd button on game startup to get into the configuration menu, if possible, and see if there are any workarounds there you can enable which may help.

Try doing a clean boot and then test the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock. 

Try creating a new OSX administrator user account (either with the Documents folder in the default location, or on a different drive, if available), switch to that account and try starting the game from there to test. 

Try exiting out of GameCenter and starting the game directly from the app container. 

Try browsing to the ‘/Users/<UserName>/Library/Containers/com.larian.dos2/Data/Documents/Larian Studios' folder and rename the 'Divinity Original Sin Enhanced Edition' folder. This folder contains the saved games, 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 OSX user account would effectively do the same thing. Try renaming the folder, but if you don't have any saved games that isn't an issue. After that, extract the replacement folder from this download into your '/Documents/Larian Studios' folder, and see if that will let you start the game. The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset, which you can change in the options (manually, or hit autodetect) if this gets the game to start. If that doesn't help, you can delete the downloaded My Documents D:OS EE folder. 

If you are playing on a system with dual graphics, in the System Preferences, Energy Saver section, try turning off Automatic Graphics Switching, and see if there is any difference if you try starting the game on the integrated graphics rather than discrete.