Flow: Error during WebSocket handshake: Unexpected response code: 501

[tstsogoo]

WebSocket connection to 'ws://localhost:8080/?v-r=push&v-uiId=0&v-pushId=48c8820b-1bde-4a5c-a2de-e703144b5d39&X-Atmosphere-tracking-id=0&X-Atmosphere-Framework=2.3.2.vaadin1-javascript&X-Atmosphere-Transport=websocket&Content-Type=application/json;%20charset=UTF-8&X-atmo-protocol=true' failed: Error during WebSocket handshake: Unexpected response code: 501
WrappedWebSocket @ VM123:164

flow 10.0.0.beta3
jdk 9
Spring boot 2.0.0
Chrome 65.0.3325.181

@HtmlImport("styles/shared-styles.html")
@Route("")
@Theme(Lumo.class)
@BodySize(height = "100vh", width = "100vw")
@Push(transport=Transport.WEBSOCKET)
public class MainView extends VerticalLayout {
private Div entryContainer;
....

        ...
new Thread(() -> {
            Div divEntry = new Div();
            divEntry.setText(entry.toString());
            UI ui = this.getUI().get();
            ui.access(() -> entryContainer.add(divEntry));
        }).start();

[Artur-]

Spring? Spring Boot? Example code? Browser?