Network Events/Alarms becomes slow and sometime hangs when i access Network Events/Alarms. I am seeing the below error in logs. Please suggest the fix if anyone knows about this issue.

[12 Jul 2016 05:30:47:283] -INFO  -SYS_ERR: [ERROR] UrlProcessor - Security Exception:  <java.lang.SecurityException: CSRF Security Error (see server log for details).>java.lang.SecurityException: CSRF Security Error (see server log for details).
        at org.directwebremoting.dwrp.BaseDwrpHandler.checkNotCsrfAttack(BaseDwrpHandler.java:81)
        at org.directwebremoting.dwrp.BasePollHandler.handle(BasePollHandler.java:111)
        at org.directwebremoting.servlet.UrlProcessor.handle(UrlProcessor.java:118)
        at org.directwebremoting.servlet.DwrServlet.doPost(DwrServlet.java:144)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:432)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:541)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
        at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
        at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:745)



java.lang.NullPointerException
        at com.adventnet.nms.startnms.AcClient.deRegisterForACCUpdate(AcClient.java:515)
        at com.adventnet.nms.webclient.common.DynamicUpdateServlet.sessionDestroyed(DynamicUpdateServlet.java:192)
        at org.directwebremoting.impl.DefaultScriptSessionManager.fireScriptSessionDestroyedEvent(DefaultScriptSessionManager.java:409)
        at org.directwebremoting.impl.DefaultScriptSessionManager.invalidate(DefaultScriptSessionManager.java:315)
        at org.directwebremoting.impl.DefaultScriptSession.invalidate(DefaultScriptSession.java:138)
        at org.directwebremoting.impl.DefaultScriptSession.invalidateIfNeeded(DefaultScriptSession.java:414)
        at org.directwebremoting.impl.DefaultScriptSession.getAttribute(DefaultScriptSession.java:72)
        at com.adventnet.nms.webclient.fault.alarm.WebACCListenerImpl$UserScriptSessionFilter.match(WebACCListenerImpl.java:162)
        at org.directwebremoting.impl.DefaultTaskDispatcher.dispatchTask(DefaultTaskDispatcher.java:45)
        at org.directwebremoting.Browser.withAllSessionsFiltered(Browser.java:88)
        at com.adventnet.nms.webclient.fault.alarm.WebACCListenerImpl$WebUpdateHandlerRunnable.run(WebACCListenerImpl.java:180)
        at com.adventnet.management.scheduler.WorkerThread.run(WorkerThread.java:82)


web.xml:

<servlet>
  <servlet-name>dwr-invoker</servlet-name>
  <servlet-class>org.directwebremoting.servlet.DwrServlet</servlet-class>
  <init-param>
     <param-name>debug</param-name>
     <param-value>true</param-value>
  </init-param>
  <init-param>
     <param-name>activeReverseAjaxEnabled</param-name>
     <param-value>true</param-value>
  </init-param>
   
     
  <load-on-startup>3</load-on-startup>
</servlet>

<servlet-mapping>
  <servlet-name>dwr-invoker</servlet-name>
  <url-pattern>/dwr/*</url-pattern>
</servlet-mapping>

<servlet>
    <servlet-name>DynamicUpdateServlet</servlet-name>
    <servlet-class>com.adventnet.nms.webclient.common.DynamicUpdateServlet</servlet-class>
    <load-on-startup>3001</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>DynamicUpdateServlet</servlet-name>
    <url-pattern>/servlets/DynamicUpdateServlet</url-pattern>
</servlet-mapping>