As JonD stated InstantSupport relies on the "Initiate Connection" popup window from the UltraVNC server.
I've done some testing since people were having problems with this and it appears that unless the WAN IP is set to a DNS name it won't appear.
Could you let me know if you are using a physical address (ex: 192.168.1.50) or a DNS name (ex: repeater.no-ip.com) for the WAN address?
I've been doing some sockets coding so in the future InstantSupport will be able to check for the repeaters existence by itself instead of relying on UltraVNC errors.
Great mod in the mean time! Open source is awesome for stuff like this.

ChunkVNC - Free PC Remote control with the Open Source UltraVNC wrapper InstantSupport!