Public Folder Migration. Exchange 2007 to Exchange 2013

I'm in the process of migrating all Public Folders from Exchange 2007 SP3 over to Exchange 2013 SP1.

When I start the migration process, I run >Get-PublicFolderMigrationRequestStatistics

I keep getting a 'TransientFailure' at %15

When looking at the report, I see this entry "Transient error MapiExceptionNotFound has occurred. The system will retry (4/60)"

I can't seem to find any solution. Can anyone point in the right direction?

Thank you in advance.

Martin.