Yes, please disable anti-virus, or at least add an exception, to see if that is causing the problem.
Also shut down any other non-essential programs, especially any backup programs, etc, that may be monitoring the Documents folder.
If you start a new game, can you save and load ok? Moving a bit from the initial character position can trigger an auto save, which might overwrite one from your current playthrough. If you don't already have an honour mode difficulty save, starting one would avoid that problem, since it uses a separate, single save slot (in the difficulty list, after selecting Tactician there is a choice between Regular and Honour).
If a new game also has a problem, does it help if you create a new profile, start a new game and save/load?
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 browsing to the '/Users/<UserName>/Library/Containers/com.larian.dos2/Data/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 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 verifying local files:
Terminal
cd /Applications
codesign -vvv Divinity\ -\ Original\ Sin\ 2.app
For the first command, if you have the game installed to a different location, 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 2.app: valid on disk
Divinity - Original Sin 2.app: satisfies its Designated Requirement
If a new game is fine, please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc), and if older saves are also ok, your most recent working save and oldest where you can no longer save. If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file 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.
