Occasionally unable to delete "Directory on Destination"

Message on the log is:"Directory on Destination cannot be deleted: The directory is not empty"

Happens on latest and previous (at least) versions of SyncBack SE with source Windows 7 Pro 64-bit, destination Windows 7 Pro either 32-bit or 64-bit, when performing "Mirror Right".

Offending destination directories and any contents can be deleted 'manually' without any problem.
This happens after source directories have been modified.

Any suggestions, please?