In case anyone every experiences this - their tech support got back to me (at last) and this solution worked:
What you should do is backup your Address Book data and reset the sync history:
1. Go to Address Book and choose 'File' menu and then 'Backup database'.
2. Still in Address Book choose 'File and then 'Revert to database backup' and choose the backup you just made.
3. In Missing Sync, click on the 'Help' menu and select 'Reset Sync History'
4. De-select other plugins and leave only 'Contacts' selected
5. Click the 'Reset Sync History' button.
Now try the sync again.