Vaadin 7 session keeps expiring automatically

I've got this app hosted with AWS; it normally works well most of the times but today I noticed a wiered problem - the session keeps expiring itself intermittently rendering the app useless. The problem then dissapears after some minutes.

On using Chrome's Inspector I got this message "WebSocket connection to 'ws://plus.sematime.com/PUSH?v-uiId=0&v-csrfToken=73284de4-7a43-4813-b428-8b8c01546b75&X-Atmosphere-tracking-id=0&X-Atmosphere-Framework=2.2.13.vaadin3-jquery&X-Atmosphere-Transport=websocket&X-Atmosphere-TrackMessageSize=true&Content-Type=application/json;%20charset=UTF-8&X-atmo-protocol=true' failed: Error during WebSocket handshake: Unexpected response code: 501"

I have also attached a screenshot of the same. 

I need some assistance in fixing this issue. Anyone?