Hi, Contrary to other servers (Tomcat, JBoss AS 7/EAP 6, GlassFish and probably all others), Undertow immediately performs a forward when HttpServletResponse#sendError() is invoked instead of waiting until all servlets and filters of the application are finished. In case the error page happens to be a Facelets page, this forward causes the FacesServlet to be invoked once again, basically creating another FacesContext inside the same thread. This overrides the previous FacesContext. After that is released, the "original" FacesContext is never put back. In OmniFaces we created a workaround for this, see https://github.com/omnifaces/omnifaces/commit/b9badb86cb2112fd87485e060db... Unfortunately, this OmniFaces workaround isn't bulletproof. In case of an HTTP 400, PrimeFaces loses its RequestContext as well and OmniFaces can't help setting it back. This results in exceptions like the following: java.lang.NullPointerException at org.primefaces.context.PrimeFacesContext.release(PrimeFacesContext.java:26) at javax.faces.webapp.FacesServlet.service(FacesServlet.java:665) Although not confirmed, this behaviour from Undertow is the likely cause of a large number of other exceptions as well. Kind regards, Arjan Tijms _______________________________________________ undertow-dev mailing list undertow-dev(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/undertow-dev