Websocket messages sometimes lost

Websocket messages are not processed in very rare cases. I have a simple onMessage method:

@OnMessage
public void onMessage(String data, final Session session, @PathParam("windowId") final String windowId) {
logger.debug("Received message from Client: "+data);
...further processing...
}

This code works 90% of cases on localhost, if I push websocket through port forward (socat in virtual machine) so that I could capture the traffic in wireshark - the problem disappears. But if connecting to localhost, some messages are still not processed. This I see only in my logs when the message is considered to be lost:

2014-12-18 15:53:35,846 DEBUG [io.undertow.websockets.core.request] (default I/O-3) Invoking receive listener
2014-12-18 15:53:35,846 DEBUG [io.undertow.websockets.core.request] (default I/O-3) UT025003: Decoding WebSocket Frame with opCode 1

If it works I see

2014-12-18 15:53:35,781 DEBUG [io.undertow.websockets.core.request] (default I/O-3) Invoking receive listener
2014-12-18 15:53:35,781 DEBUG [io.undertow.websockets.core.request] (default I/O-3) UT025003: Decoding WebSocket Frame with opCode 1
2014-12-18 15:53:35,783 DEBUG [classname] (default task-55) Received message from Client: <data>