UltraVNC

That's what I was trying all the time. Finally I manually entered it into the custom.txt and finally the client connected to the right place. BUT...when the connection was made (which took about one minute each time), the viewer window popped up and disappeared again - the server read "Connection ...

I totally agree with you as this is the most-wanted option for me. I am missing it since I was using this quite confidently with SC.