Outlook contact folder 'one or more items in the folder you synchronized do not match - to resolve the conflicts, open the items, and they try this operation again'

Outlook 2016 client on Exchange 2010 server fully patched.
User has a contacts public folder, within that folder is another contact folder called clients.
When adding or modifying a contact in that folder, when you go to save the following error appears:

Microsoft Outlook
One or more items in the folder you synchronized do not match.  To resolve the conflicts, open the items, and then try this operation again.

There is no obvious conflict within the clients folder or the parent contacts folder.

If I click folders and look at Synch Issues, and its subfolders, conflicts, local failures, server failures, those are all empty.

This is cache mode, so I tried creating a new profile with cache mode and another with cache mode off, same error in the same place.

Does anyone know if there is a log that points to whatever conflict Outlook thinks there is?  I see no indication of that, but no changes can be made.  This is new behavior in the last week or so - previous any changes saved OK.