JBoss and httpd are failing to join together due to "client denied by server configuration"

Issue

• JBoss and httpd are failing to join together through mod_cluster.
• From the EAP 5 logging, we see:

ERROR [org.jboss.modcluster.mcmp.impl.DefaultMCMPHandler] (ContainerBackgroundProcessor[StandardEngine[jboss.web]]) Error [null: null: {4}] sending command INFO to proxy <IP>:<Port>, configuration will be reset

• Or on EAP 6:

ERROR [org.jboss.modcluster] (ContainerBackgroundProcessor[StandardEngine[jboss.web]]) MODCLUSTER000042: Error null sending INFO command to <IP>:<Port>, configuration will be reset: null

• Or on EAP 7:

ERROR [org.jboss.modcluster] (UndertowEventHandlerAdapter - 1) MODCLUSTER000042: Error null sending INFO command to machinename/x.x.x.x:6666, configuration will be reset: null

• And on the Apache side:

[error] [client 127.0.0.1] client denied by server configuration: /