If you are receiving this message via the Spacewalk web interface the explanation can be found by reading this message from the mailing list.

To resolve the issue, install the newer Spacewalk certificate, if you have already upgraded to Spacewalk 2.2 or newer, run this command (service/server restart is not required)

{{{ #!sh $ sudo rhn-satellite-activate --rhn-cert /usr/share/spacewalk/setup/spacewalk-public.cert --disconnected }}}

If you are still running an older version before Spacewalk 2.2, download the certificate and install (again, no need to restart the server/service)

{{{ #!sh $ wget 'https://raw.githubusercontent.com/spacewalkproject/spacewalk/master/branding/setup/spacewalk-public.cert' -O spacewalk-public.cert $ sudo rhn-satellite-activate --rhn-cert spacewalk-public.cert --disconnected }}}