In our application sometimes (without any special reason) this exception occurs.
The last time first a overlay popup was shown saying "Internal error" (maybe different wording) and after some seconds this stacktrace appeared:

Java Stacktrace

java.lang.NullPointerException
at org.icefaces.impl.event.WindowAndViewIDSetup$1.encode(WindowAndViewIDSetup.java:66)
at org.icefaces.impl.event.UIOutputWriter.encodeBegin(UIOutputWriter.java:27)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1777)
at javax.faces.render.Renderer.encodeChildren(Renderer.java:168)
at org.icefaces.impl.renderkit.RendererWrapper.encodeChildren(RendererWrapper.java:49)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
at com.icesoft.faces.renderkit.dom_html_basic.DomBasicRenderer.encodeParentAndChildren(DomBasicRenderer.java:341)
at com.icesoft.faces.renderkit.dom_html_basic.GroupRenderer.encodeChildren(GroupRenderer.java:79)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
at com.icesoft.faces.renderkit.dom_html_basic.DomBasicRenderer.encodeParentAndChildren(DomBasicRenderer.java:341)
at com.icesoft.faces.renderkit.dom_html_basic.GroupRenderer.encodeChildren(GroupRenderer.java:79)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.encodeRecursive(HtmlBasicRenderer.java:304)
at com.sun.faces.renderkit.html_basic.GroupRenderer.encodeChildren(GroupRenderer.java:105)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1779)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1782)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1782)
at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:402)
at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:125)
at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:594)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.filters.ExpiresFilter.doFilter(ExpiresFilter.java:1179)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)