first enable Desktop Sharing on Ubuntu 16.04 host
and protect it with password.
Ubuntu is running Vino VNC Server and UltraVNC does not support Vino encryption. You have to execute without restart and sudo(!) the followinf command
$ gsettings set org.gnome.Vino require-encryption false
Enjoy!
PS: I did not test the settings with TightVNC or RealVNC, but expecting also to be working solution
