The 'nxclient --monitor' process doesn't start when NoMachine is installed on Ubuntu with Wayland enabled. As a consequence users cannot connect to the physical desktop or create new virtual desktops (when supported) and get message 'no available sessions'.

This problem has been reproduced on Ubuntu 17.10, 18.04 and 18.10. It was not reproduced on Fedora 28.

As a workaround:

1) disable Wayland in the display manager configuration.

E.g. in case of GNOME desktop, add the following key to /etc/gdm/custom.conf:

WaylandEnable=false

and restart gdm:

systemctl restart gdm

2) and restart NoMachine server (sudo /etc/NX/nxserver --restart)