Earlier this month (Jnauary) Microsoft posted a critical security update to TCP/IP that will result in the file TCPIP.SYS being replaced. The patch fixes a remote code execution flaw in TCP/IP for Windows XP, 2000, Server 2003 and Vista. If you use Automatic Updates, WSUS or any other update tool you have probably already received this update.
The update is MS08-001 (KB941644).
Since TCPIP.SYS is replaced, this will roll the half-open connection limit back to 10 connections, resulting in an error from VNCScan.
If you're using Windows XP SP2 or Server 2003, you can patch this back to a higher limit using the tool available on this site's Downloads page.
If you're using Vista, you're probably out of luck as I'm not aware of any way to change the limit in Vista.
http://www.microsoft.com/technet/security/bulletin/ms08-001.mspx
(By the way, Steve, the "video" on your support page on how to do this shows nothing execept a mouse being moved around the screen.)