This issue might be related to UNDERTOW-358

This is how I create a websocket client endpoint:

ClientEndpointConfig config = ClientEndpointConfig.Builder.create().build();
config.getUserProperties().put("io.undertow.websocket.CONNECT_TIMEOUT", 10);
WebSocketContainer socketContainer = ContainerProvider.getWebSocketContainer();
MyClient client = new MyClient(); // extends Endpoint
session = socketContainer.connectToServer(client, config, getEventStreamURI());

With this code the @OnMessage method of my client will never get called. Every time a message is sent I see "[XNIO-1 I/O-1] DEBUG io.undertow.websockets.core.request - UT025003: Decoding WebSocket Frame with opCode 1" in the log but that's it.
However, if I create the client without passing the config parameter to connectToServer() it works perfectly fine (but then I won't get notified when the server goes down so I have to provide the timeout parameter).