P: Sync Yields "bad argument #1 to '?' (number expected, got nil)" Error Messages

Within the last several weeks, I have started to get errors when trying to sync from LR Classic.  Some problem images have been successfully synced for months, and others are new.  The problems seemed to begin when I removed some photos from All Photos in LR CC.  In the upper left corner, I see "Syncing 3 photos", though the number changes.  In Preferences, Sync Activity, the images have had the message "bad argument #1 to '?' (number expected, got nil)", and the Remove from All Synced Photos option doesn't seem to do anything.
Is there some way I can reset the sync environment?  Would that be a solution? Thanks.