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.
The Apache Server need to be be reconfigured so as not to block access to the UMP Control Panel page.