While I don't have an answer for your problem, I'd like to report another problem related to Quiz Restore found in version 1.5.3+.
User attempts are not restored during the process. A message that this will be implemented in 1.5.1 is displayed.
For now I would not recommend relying on Backup & Restore for archival or disaster recovery purposes, instead only use it for creating or resetting a new (no user data) course. Reported as bug #4608.
Another issue is complete loss of Glossary objects, reported as bug #4606.