Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Beta Error launching mac version from Steam

You're probably using global disk encryption, because that's the only thing that comes to my mind.

Go to Steam->Preferences->Downloads.
Click on Steam Library Folders
Create SteamLibrary inside your Applications folder f.ex.
Delete Divinity Original Sin game from your library
Install it again and specify /Applications/SteamLibrary as a place of download

Run it.