ERROR "UT000066: Incorrect magic number XXXX for AJP packet header" occured in EAP 7

Issue

The following ERROR message was shown in server.log in JBoss EAP 7. What does it mean:

ERROR [io.undertow.request] (default I/O-12) UT005001: An exception occurred processing the request: java.io.IOException: UT000066: Incorrect magic number 18245 for AJP packet header
    at io.undertow.server.protocol.ajp.AjpRequestParser.parse(AjpRequestParser.java:189)
    at io.undertow.server.protocol.ajp.AjpReadListener.handleEvent(AjpReadListener.java:166)
    at io.undertow.server.protocol.ajp.AjpOpenListener.handleEvent(AjpOpenListener.java:131)
    at io.undertow.server.protocol.ajp.AjpOpenListener.handleEvent(AjpOpenListener.java:51)
    at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
    at org.xnio.ChannelListeners$10.handleEvent(ChannelListeners.java:291)
    at org.xnio.ChannelListeners$10.handleEvent(ChannelListeners.java:286)
    at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
    at org.xnio.nio.QueuedNioTcpServer$1.run(QueuedNioTcpServer.java:129)
    at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:588)
    at org.xnio.nio.WorkerThread.run(WorkerThread.java:468)