Word can't open some files after Update to MS 365

1. Verify that Microsoft 365 is native to Apple Silicon
Open Finder > Applications, right-click Microsoft Word, and select Show Profile.
Check that “Use Rosetta” is not checked. If checked, uncheck and restart Word (make sure ARM native version is installed).
 To reset document permissions on a Sonoma system
Go to System Settings > Privacy & Security > Documents & Folders.
Verify that Microsoft Word has full disk access to the desktop, downloads folder, etc.
Addendum: Sonoma's sandbox is more restrictive and requires manual authorization.
Disable iCloud Drive Sync Conflicts
2. Temporarily disable iCloud synchronization test: Go to System Settings > Apple ID > iCloud and disable iCloud Drive.
Try to save the file in Word to the desktop and see if it succeeds.
If successful, you need to troubleshoot the iCloud synchronization path conflict (file path is occupied).
3. Use Terminal to repair file system permissions
Open Terminal and type:
bash
sudo chmod -R 755 ~/Documents
sudo chown -R $(whoami) ~/Documents
In Word Recovery Mode, press Cmd+A to select all the text and change the font temporarily to the system default font (e.g. San Francisco).
Save and check whether it is normal, if successful, then the original document used M1 incompatible with the old fonts.
Completely uninstall the old version of Office
4. Download the official Microsoft uninstallation tool **Microsoft Office Removal Tool**.
Run it and select “Remove All Office Licenses”, reboot and download the ARM native installer from the official website.
Create a temporary local user test environment
Go to System Settings > Users & Groups and create a new temporary administrator account.
Log in to the new account and run Word to try to open the document. If successful, the original account configuration is corrupted and you need to migrate the data.