I’m not able to tell you which lines to look at without looking into to log. Therefore you’d need to post it here or somewhere and send me the link.
If you want to debug it yourself, then look for MoveItems request of the Outlook user and check if multiple items are being moved. Search for the not moved email’s subject, so that you can find out the item’s ServerEntryId. Then you search for the occurrences of the ServerEntryId in the log and so on.