>Вся проблема заключается в отсутствии описания permision на оператор RESTORE
IMHO в BOL все написано

"Permissions
If the database being restored does not exist, the user must have CREATE DATABASE permissions to be able to execute RESTORE. If the database exists, RESTORE permissions default to members of the sysadmin and dbcreator fixed server roles and the owner (dbo) of the database.

RESTORE permissions are given to roles in which membership information is always readily available to the server. Because fixed database role membership can be checked only when the database is accessible and undamaged, which is not always the case when RESTORE is executed, members of the db_owner fixed database role do not have RESTORE permissions.
"

Возможно еще, что
"During the restore, the specified database must not be in use ."