The first things that comes to my mind when the login dialog does not show is, that you might use the Java plugin at your browser side. If so, please go to the control panel -> Java Plugin and uncheck the plug-in for both browsers as their default Java runtime. Make sure you see a dialog popping up daying 'you just changed your browser settings' or something similar. Restart your browser and try again.
If that does not work, I need more information that probably will be given by the DiagnosticsServlet.