Vaadin 7.3.6 - Push with WebSocket fails. Fallback to long polling not reli

My Application runs in WebSphere 8.5.5 / Liberty Profile with vaadin 7.3.6. Without Liberty Profile deploying the ear on WebSphere I get the same results.

In my web.xml I have org.atmosphere.cpr.broadcasterCacheClass as , org.atmosphere.cpr.SessionSupport as listener class and true

The main Vaadin UI Class is annotated buy @push.

When I log in into the session on firefox with firebug the push Connection with websocket fails, telling me (Firebug console):

NetworkError: 501 Not Implemented - http://<url>/app/PUSH/?v-uiId=1&v-csrfToken=<token>&X-Atmosphere-tracking-id=0&X-Atmosphere-Framework=2.1.5.vaadin4-jquery&X-Atmosphere-Transport=websocket&X-Atmosphere-TrackMessageSize=true&X-Cache-Date=0&Content-Type=application/json;%20charset=UTF-8&X-atmo-protocol=true"
Websocket closed, reason: Connection was closed abnormally (that is, with no close frame being sent).
Websocket failed. Downgrading to Comet and resending.

Then it falls back to long polling making get requests periodically. Push requests to the application work for some time, but normally when you leave the client alone a few minutes a notification Popups rises up telling

"Communication problem. Take note... Push connection usinng websockets failed" How can I make websockets work, or how can I make the long polling mechanism more reliable?

Thank you in advance!

On startup get the message:

[ERROR   ]
Not a File: file:\C:\<serverdir>\apps\ServiceCockpitEAR.ear\ServiceCockpitWeb.war\WEB-INF\lib\atmosphere-runtime-2.1.2.vaadin6.jar!\org\atmosphere\annotation

Maybe this has something to do with it?