rhn_register returns error "spacewalk.server.apacheRequest.UnknownXML : Invalid request received (function: registration.remaining_subscriptions invalid)"

Issue

• When registering a RHEL box it throws an exception as follows:

[Tue Apr  9 07:10:51 2013] rhn_register 
Traceback (most recent call last):
  File "/usr/sbin/rhn_register", line 79, in ?
    app.run()
  File "/usr/share/rhn/up2date_client/rhncli.py", line 65, in run
    sys.exit(self.main() or 0)
  File "/usr/sbin/rhn_register", line 64, in main
    ui.main()
  File "/usr/share/rhn/up2date_client/tui.py", line 1851, in main
    tui.run()
  File "/usr/share/rhn/up2date_client/tui.py", line 1758, in run
    if self._show_subscription_window() == False:
  File "/usr/share/rhn/up2date_client/tui.py", line 1716, in _show_subscription_window
    self.password)
  File "/usr/share/rhn/up2date_client/rhnreg.py", line 545, in getRemainingSubscriptions
    version)
  File "/usr/share/rhn/up2date_client/rhnserver.py", line 52, in __call__
    raise self.__exception_from_fault(f)
up2date_client.up2dateErrors.UnknownMethodException: While running 'registration.remaining_subscriptions': caught
spacewalk.server.apacheRequest.UnknownXML : Invalid request received (function: registration.remaining_subscriptions invalid)