It seems your perseverance helped us to find a bug 🙂
http://www.nomachine.com/AR10K00710
We have just edited AR10K00710 to provide instructions for
launching Xvfb as unprivileged user, we made additional
tests and we found some machines where running Xvfb as root
prevents NoMachine to be able to connect to the X server.
So please use:
‘Xvfb :0 -auth /var/run/Xvfb-0.auth -screen 0 1024x768x24 &’ instead.
Be sure that your run Xvfb as the same user who starts the dekstop
environment, otherwise the desktop will be not available.
We have opened a trouble report on that topic:
http://www.nomachine.com/TR11K04058