A SOLUTION!!
I posted recently to say that i was having the problem. I chipped away at mine, and eventually worked it out.
To start with, the specific error i was receiving was:
An error occurred retrieving the WMI Repository data directory.
The WMI Repository could not be backed up.
The event viewer showed a Warning and ID 8, Source: winmgmt. I searched all online resources and found nothing, until I altered my search one day and found the relevant info on www.eventid.net.
The solution for me was to check the permissions to the %systemroot%\system32\wbem\repository directory. Ensure that the relevant admins, and of course the Backup Exec Service Account has permissions to that directory. If you suspect corrupt files, remove everything from that directory (relocate rather than delete), and then Stop/Start the WMI services, and/or reboot the server. The files will be re-created.
The key for me was that i was getting semi-persistent WMI advice in my Event Log, but when i altered permissions this changed. The following evenings backup went without a hitch.
Good luck to you all, i hope this info can assist some of you.