A filter or servlet of the current chain does not support asynchronous oper

We are running Vaadin 7.5.4 on Tomcat 8.0.23 and we sometimes get browsers stuck with this exception (this time it was the current version of Safari 8.0.8).  We're not sure why as the webapp otherwise appears to run fine "most of the time."  Pushmode is automatic, though we don't use push much by design yet.

Does that error means a filter/servlet in the chain I can see listed in the stack trace is the culprit?  Is there any way to identify which?  The only one we set "ourselves" is the SetCharacterEncodingFilter for UTF-8.  But that filter occurs on every request, so if it had an issue, wouldn't it always have an issue and not just randomly from time to time?  (Not sure why that filter shows up twice -- can a filter be installed twice?)

VaadinSession.ErrorHandler.error() reports:

java.lang.IllegalStateException: A filter or servlet of the current chain does not support asynchronous operations.
        at org.apache.catalina.connector.Request.startAsync(Request.java:1610)
        at org.apache.catalina.connector.RequestFacade.startAsync(RequestFacade.java:1037)
        at javax.servlet.ServletRequestWrapper.startAsync(ServletRequestWrapper.java:392)
        at org.atmosphere.cpr.AtmosphereRequest.startAsync(AtmosphereRequest.java:723)
        at org.atmosphere.container.Servlet30CometSupport.suspend(Servlet30CometSupport.java:93)
        at org.atmosphere.container.Servlet30CometSupport.service(Servlet30CometSupport.java:68)
        at org.atmosphere.cpr.AtmosphereFramework.doCometSupport(AtmosphereFramework.java:2075)
        at com.vaadin.server.communication.PushRequestHandler.handleRequest(PushRequestHandler.java:226)
        at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1408)
        at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:351)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:614)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
        at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:617)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:668)

Try to switch to longpoll mode.