Yum fails on RHN Proxy clients with error "Error communicating with server. The message was: Internal Server Error"

Issue

• While running yum commands on clients registered to RHN Proxy, the following error is received:

# yum list updates
Loaded plugins: aliases, changelog, product-id, protectbase, rhnplugin, search-disabled-repos, subscription-
              : manager, verify, versionlock
There was an error communicating with RHN.
Red Hat Satellite or RHN Classic support will be disabled.
Error communicating with server. The message was:
Internal Server Error