I have had this problem on Backup Exec 12 Rev 1364 SP 5 and Exchange 2003 using GRT
The error details are:
V-79-57344-33928 - Access is denied.
Access denied to directory Mailbox Store (Exchange server name) . <mailbox name> [mailbox name]. Top of Information Store.
Final error: 0xe0008488 - Access is denied
Final error category: Security Errors
After checking all the obvious settings and found them to be OK, I have followed the solution of Duplicating the database to disk but once completed (successfully), I can not see the individual mailboxes to restore. I can only see the stores.
I have tried re-cataloging but a message says it is already catalogued.