Confirm the correct package:

 rhn-org-httpd-ssl-key-pair-MACHINE_NAME-VER-REL.noarch.rpm 

is installed on the RHN Proxy Server and the corresponding rhn-org-trusted-ssl-cert-*.noarch.rpm or raw CA SSL public (client) certificate is installed on all client systems.

Verify the client systems are configured to use the appropriate certificate.

If using one or more RHN Proxy Servers, ensure each Proxy's SSL certificate is prepared correctly. If using the RHN Proxy Server in conjunction with an RHN Satellite Server the Proxy should have both its own server SSL key-pair and CA SSL public (client) certificate installed, since it will serve in both capacities. Refer to the SSL Certificates chapter of the RHN Client Configuration Guide for specific instructions.

If the RHN Proxy Server is connecting through an HTTP Proxy, make sure the URL listed is valid. For instance, the HTTP Proxy URL field should not contain references to protocols, such as http:// or https://. Only the hostname and port should be included in the form hostname:port, such as your-gateway.example.com:8080.

Make sure client systems are not using firewalls of their own blocking required ports, as identified in Section 2.4, “Additional Requirements”.