Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the Documents folder?
Try browsing to the '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/LevelCache' folder and delete the contents. A corrupt file there can cause problems when saving or loading.
Try verifying local files: in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.
Try creating a new profile, start a new game, go part way through the tutorial section and save/load to see if this still happens.
If this happens in a new profile, 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.
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.
If that doesn't help, please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc). If you are using any mods, also include a list of those enabled and/or the modsettings.lsx from your profile folder.
Each save is a folder in the '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/PlayerProfiles/<ProfileName>/Savegames/Story' folder.
In Finder you can zip a file or folder by right clicking on it/them and selecting Compress.
