Backup Exec failing on Exchange backup. Error is The following error was returned when opening the Exchange Database file: ''

Hello,

    I am using Backup Exec 2010 on a SBS 2008 server to back it up.  Lately, I have been getting an error when it backs up Exchange which is:

V-79-57344-759 - Unable to complete the operation for the selected resource using the specified options.  The following error was returned when opening the Exchange Database file:  '<n/a>'
 

The log states that the store was backed up but this error still occurred and it appears to have to do with backing up individual mailboxes as there is no Mailbox Database section in the catalog for the failed backups.  It doesn't happen every backup either, but maybe a couple of times a week.  I have Exchange SP3 installed and updates for Backup Exec also.  Can anyone tell me why this is occuring and why there is no error message except '<n/a>'?

Thank you