Accessing the UMP Control Panel->Users or any other link under Control Panel causes this message to occur:
"Forbidden - You don't have permission to access /group/control_panel/manage on this server."
Whether the user is logged in as administrator/superuser makes no difference.
It was discovered that there was a separate, second Apache Server installed on the UMP machine acting as a reverse proxy and it was interfering with access to the Control Panel page.
Resolution: The Apache Server need to be be reconfigured so as not to block access to the UMP Control Panel page.