After upgrading from Ubuntu 12.04 to Ubuntu 14.04, users get a black screen when creating a new virtual desktop when:

- A NoMachine version previous than 4.3.x (or NX Server 3.5.0) is installed.

- A GNOME fallback desktop environment is set.

Note also that this article applies to virtual desktop sessions only.

As explained in this bug report  for Ubuntu 14.04, also the GNOME fallback now requires graphic acceleration:

https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/1251281

This means that such desktop environment is no longer working with NoMachine versions previous than 4.3 because these earlier versions of the software don't  have a full support for 3D graphics.

We advises our users to upgrade to the last NoMachine version (we added support for hardware acceleration and Unity 3D since version 4.3). 

To use a lightweight desktop environment instead, please read here:

https://www.nomachine.com/AR07K00676

Using a lightweight environment like MATE or Xfce is a solution also for NX 3.5.0 users.