I have previously used Bianca KDE at home to connect to various XP machines at work using Krdc. Now I've upgraded at home to both Cassandra and Daryna, I can no longer connect properly - I get prompted for a password, the remote screen opens but is completely black. As a test I have tried connecting from home with XP and PCLinux OS live CD and they both work fine.
I can also connect between the Daryna and Cassandra machines across my LAN at home without any problems.
Seems that when you leave the LAN something goes wrong. VNC is not really for LAN use, but check if there are settings for use on different networks in VNC, and perhaps there is a forum or FAQ at VNC that helps. I don't use VNC so I can only be of limited help
Don't fix it if it ain't broken, don't break it if you can't fix it