Getting "wrong magic number" exception with WF 9 and mod_ajp (apache)

Periodically getting the following exception..

2015-08-06 08:28:55,937 ERROR [io.undertow.request] (default I/O-2) UT005001: An exception occurred processing the request: java.lang.IllegalStateException: Wrong magic number
at io.undertow.server.protocol.ajp.AjpRequestParser.parse(AjpRequestParser.java:188)
at io.undertow.server.protocol.ajp.AjpReadListener.handleEvent(AjpReadListener.java:164)
at io.undertow.server.protocol.ajp.AjpReadListener.handleEvent(AjpReadListener.java:54)
at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92) [xnio-api-3.3.1.Final.jar:3.3.1.Final]
at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66) [xnio-api-3.3.1.Final.jar:3.3.1.Final]
at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:88) [xnio-nio-3.3.1.Final.jar:3.3.1.Final]
at org.xnio.nio.WorkerThread.run(WorkerThread.java:539) [xnio-nio-3.3.1.Final.jar:3.3.1.Final]