We did realize this issue when we were trying to link a new ticket to another one. The target ticket number does appear on 2 separate tickets. Both of them are about the same issue. One was created by the customer through the help center and the one was created when an employee did forward it to helpdesk.

Also Desk did not allow as to link to the target ticket at first. The system displayed the error message which was informed to you yesterday by Harry Brant on the "Linking Tickets" post. Later on I was able to open the parent-child widget on the sidebar.