The CONNECTED:STREAM-SENSING should not last more than a few seconds, what do you mean with "long time"?

As per what is happening, probably the client heuristically decided to disable websockets, that's why even if you disconnect-reconnect you can't get a WebSocket connection. A server going down, may cause such behavior if it goes down when the websocket is open but no data was yet exchanged. If you attach a server log (with at least LightstreamerLogger.requests at INFO level) I may be able to verify if the behavior is expected or not.