Today I have checked with a xfce 4 system with DefaultDesktopCommand “/usr/bin/startxfce4” and also in this case the result is a black screen.
I guess the problem is with the dbus, since in the first case (LXQt) I can launch pcmanfm-qt in a terminal window only with the command “dbus-launch pcmanfm-qt” and in the second case (xfce 4) there are some popup errors about dbus.
Maybe this is not possible with NoMachine free?