Sometimes, when accessing a machine with Ubuntu 12.04 (or newer) running NXServer 3.5 from a system running NXClient 3.5 or OpenNX, all you can see is a screen with a desktop picture but no panels or menus.
In this case it is necessary to edit the file /usr/NX/etc/node.cfg on the server in order for it to default the virtual desktop to 2D, without effects.
Almost at the end of the file change the line CommandStartGnome to one of the following:
CommandStartGnome = "/usr/bin/gnome-session --session=ubuntu-2d"
For Gnome Classic:
CommandStartGnome = “/usr/bin/gnome-session --session=gnome-classic”