[Satellite 5] Unable to activate RHN Proxy

Issue

When trying to activate RHN Proxy, following error is displayed:

[root@hostname ~]# rhn-proxy-activate --version 5.7 --non-interactive

There was a problem activating the Spacewalk Proxy entitlement:
ERROR: upon entitlement/activation attempt: unknown error - <Fault -1: u'redstone.xmlrpc.XmlRpcFault: unhandled internal exception: ERREUR: -20235 : (channel_family_no_subscriptions) - Channel family subscription count cannot exceed maximum membership\n  Where: SQL statement "SELECT rhn_exception.raise_exception(\'channel_family_no_subscriptions\')"\nfonction PL/pgsql rhn_channel.subscribe_server(numeric,numeric,numeric,numeric,numeric), ligne 105 \xe0 PERFORM'>