Websocket gets closed immediately

Using the following code in Espruino Web IDE:

var WebSocket = require("ws");
var ws = new WebSocket( 'mydomain.com', { port:8822 } );

ws.on('open', function() {
  console.log("Connected to server");
});

ws.on('message', function(msg) {
  console.log("MSG: " + msg);
});

ws.on('close', function() {
  console.log("Connection closed");
});

ws.on('handshake', function() {
  console.log("Handshake Success");
});

ws.on('rawData', function(msg) {
  console.log("RAW: " + msg);
});

ws.send( JSON.stringify( { text:'HHHHLLLLL' } ) );

I can see in the server logs, that the WS-connection is established and then closes within some milisecs:

12:24:06.009 [vert.x-eventloop-thread-0] INFO com.my.GatewayWSVerticle - registered WS for [11.111.111.111]
12:24:06.220 [vert.x-eventloop-thread-0] INFO com.my.GatewayWSVerticle - unregistered WS for [11.111.111.111]

If I enter ws.initializeConnection(); in the left-hand side of IDE, I'm getting the lines:

RAW: HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Handshake Success
Connection closed

If I connect to the very same server from my browser or from a z-way server @ raspi, then it works like charm.

Am I missing something or is it a WS-bug?

TIA