try this

Check to see that the service is running as a system user and not the user that
installed the service.   If you do not want the system user to do the backup,
then be sure that the user authorities for domains is correct. in our
environment there is a domain that controls the system, but some of our servers
are their own domain within the system domain.  Therefore our operators need to
be members to the machine's domain as well.

Rob