vncviewer fails to connect to x11vnc server with (xdmcp+Xvfb) virtual x session
 

I've noticed that vncviewer on an N900 fails to connect to this x11vnc server setup (just shows a black screen) but Presence VNC viewer on N900 connects with no problem. Why is that? I prefer vncviewer because of its mouse input but cannot get this to work properly with it.