You sure it's fixed?
I have a very similar problem, which happens both with an installation of 220.127.116.11 and after de-installation and re-installation of ver 18.104.22.168
The computer the VNCserver is running on has installed "Windows Server 2003 Enterprise Edition Service Pack 2", and has an activated ScreenSaver with the option enabled to ask for the credentials again after the screensaver kicks in.
The effect is, that after a certain time (POSSIBLY the time necessary for the screensaver to come up, maybe less), the VNCserver freezes, and the entire computer is locked up. Unfortunately I don't have physical access to the computer, but according to the support-guys that DO have access, the computer wasn't even reacting to input on a locally connected keyboard, and they had to do a hardware-reset. What I could see was: no response on ports 22,80,443,3389,5900 all of which SHOULD be open normally.
One thing that might (but probably shouldn't) affect the problem is, that I was also logged in via RemoteDesktop at the time I tried out the configuration of UltraVNC - oh, and UltraVNC was run as a service, when it happend, and the "mirror-driver" was not installed.
Hope this helps to track the problem,