What are your system specs (you can use the 'System Report' button from the 'About This Mac' Apple menu item, or the 'System Information' app in the utilities folder), including the OS version number, Mac model identifier and year early/mid/late indicator?
What graphics drivers are you using?

If you hold the Cmd button on game startup to get to the configuration menu, does that make a difference with the error or Alt-Tabbing?

If applicable, disable cloud support in Galaxy: this can be done either globally (click on the GOG.com button and select Settings, and then Features) or just for this game (with the game selected, click the More button and select Settings, then near the bottom unselect the Cloud saves option).

Next, try browsing to the '~/Documents/Larian Studios' folder and delete the 'Divinity Original Sin 2 Definitive 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. With cloud support enabled, Galaxy might just download the cloud copy of the graphics config file (not sure it it would sync that with no profile or saves, though).

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 and low audio quality, which you can change in the options (manually, or hit autodetect) if this gets the game to start.
If you have the same graphics issue, you can delete the replacement Documents D:OS 2 DE folder.