getAttribute: Session already invalidated

For one of my web application based on vaadin, I am using modified version of QuickTickets Dashboard project. The war is hosted on tomcat7 and it was working fine since last 6 months. All of a sudden, the application started to act up and throwing the expection pasted below. I tried different suggestion on form and over internet but nothing workes. I even tried to deploy war on fresh tomcat7 installtion without any luck.

Sep 20, 2016 11:36:23 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [GMLCPortalDashboard]
 in context with path 
[/gmlc-portal] threw exception [com.vaadin.server.ServiceException: java.lang.IllegalStateException: getAttribute: Session already invalidated]
 with root cause
java.lang.IllegalStateException: getAttribute: Session already invalidated
        at org.apache.catalina.session.StandardSession.getAttribute(StandardSession.java:1189)
        at org.apache.catalina.session.StandardSessionFacade.getAttribute(StandardSessionFacade.java:120)
        at com.vaadin.server.WrappedHttpSession.getAttribute(WrappedHttpSession.java:54)
        at com.vaadin.server.VaadinService.getSessionLock(VaadinService.java:580)
        at com.vaadin.server.VaadinService.unlockSession(VaadinService.java:649)
        at com.vaadin.server.VaadinService.findOrCreateVaadinSession(VaadinService.java:667)
        at com.vaadin.server.VaadinService.findVaadinSession(VaadinService.java:524)
        at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1402)
        at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:350)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:744)

Sep 20, 2016 12:45:00 PM com.vaadin.server.DefaultErrorHandler doDefault
SEVERE:
java.lang.NullPointerException
        at com.vaadin.ui.ConnectorTracker.removeUnregisteredConnectors(ConnectorTracker.java:341)
        at com.vaadin.ui.ConnectorTracker.cleanConnectorMap(ConnectorTracker.java:285)
        at com.vaadin.server.communication.UidlWriter.write(UidlWriter.java:304)
        at com.vaadin.server.communication.UidlRequestHandler.writeUidl(UidlRequestHandler.java:149)
        at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:98)
        at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
        at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1408)
        at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:350)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:744)