I wonder if it would save if you deleted users, custom groups, and shares.
Obviously, this may not be practical. If you delete a user, and then add them back later, that user may not be able to consume his previous user-folder (the one created upon logging into a Windows workstation).
The idea is too remove stuff from it until you're able to save 100% and then add things back (saving after each thing you add). This would answer the question "which thing is corrupting samba's ability to save". Or, doing this may lead to it working correctly. Maybe a previous version did some corrupted thing, and the updates are unable to correct that action.
Hopefully, someone will contribute a nicer solution, but since no one has yet, I thought I'd share this painful idea with you