There has been a change in behavior between the JBoss EAP server 7.1 DR2 and 7.1 DR3 builds.
The websocket-client quickstart uses a relay between a frontend WebSocket server endpoint and a backend WebSocket client endpoint. It connects to a publicly available echo server using a secured connection: wss://echo.websocket.org
I have tested this quickstart  against JBoss EAP 7.0 GA, 7.1 DR, and 7.1 DR2 and it works as expected.
When I test it using the JBoss EAP server DR3 build, I get this error:
I also tested it against a build of the current source (7.x) for JBoss EAP 7, and it fails with the same message.
Something changed between DR2 and DR3 that results in the failure of this quickstart.