Try edit config.txt in boot and add this

hdmi_force_hotplug=1
hdmi_mode=16

This seems a bit reminiscent of when I was troubleshooting dual monitors on the Pi4. The second monitor would show the splash screen, then go blank.
I had to uncomment dtoverlay=vc4-kms-v3d in /boot/config.txt
Then because I wanted each display to be the same screen rather than an extended desktop, I had to overlap Monitor 1 and 2 in the Screen Configuration, layout editor.

All, see this thread here:

www.indilib.org/forum/stellarmate/9453-f...-ubuntu-install.html

The topic starter is not using a Raspberry Pi.

We are turning around in circles here. All these problems have been solved many times over here on this forum.

It is usually a simple matter of just editing the config.txt file in the boot section of the Pi4.

Also, x11VNC works just fine. Couldn't be more straightforward.

Set the framebuffer to the size of your monitor in the config.txt file

set the hdmi_hotplug and the hdmi_mode (82 in my case for 1920x1080)

set x11vnc -storepasswd

add x11vnc -usepw -shared -forever to the Ubuntu Startup Items

That should do it.

Not a Pi. No config.txt. No "boot section"

I have been running a NUC with NoMachine for some time now. I don't recall a black screen (other than a slow startup that looks black lately).

I use the HDMI dummy plug and make sure I'm running the same release of NoMachine on both machines. I am running Debian Sid on the monitoring desktop and Kubuntu on the NUC.

Even easier on the mini-PC:

1.  Go to Synaptic, install x11vnc
2.  Go to Applications > Internet > X11VNC server
3. Click OK, then follow the instructions in the menu to start x11vnc at startup
4. Add another startup item under System > Preferences > Personal > Startup Applications to define Screen size:  xrandr --fb 1920x1080  (or the screen size you want)

Restart.  You should now be able to log into your VNC server using Remmina on Linux, Remote Desktop on the Mac, ChickenVNC on the Mac, etc. 

No dummy plug required.

These instructions are for UbuntuMATE.  I prefer that Desktop environment.  Easier to navigate than Unity.
 

OK everyone, thank you for the assistance. There is a lot of good, specific advice here to try. I've attempted several of these prior to opening this thread, but often with some slight variations found elsewhere online.

Amazon delivered my HDMI dummy plugs early, and bam -- everything works just fine with NoMachine, including re-sizing the client window triggering a resizing of the Remote Desktop. I'm up and running -- thanks for the suggestion, AirScott!

This worked for me:  

Install this dummy device<code>sudo apt-get install xserver-xorg-video-dummy</code>
Then create /etc/X11/xorg.conf with the following details:

<code>Section "Device"
  Identifier "Configured Video Device"
  Driver "dummy"
EndSection

Section "Monitor"
  Identifier "Configured Monitor"
  HorizSync 31.5-48.5
  VertRefresh 50-70
EndSection

Section "Screen"
  Identifier "Default Screen"
  Monitor "Configured Monitor"
  Device "Configured Video Device"
  DefaultDepth 24
  SubSection "Display"
  Depth 24
  Modes "1024x800"
EndSubSection

EndSection</code>

Restart

This looks quite useful. This is on a NUC or mini-PC running ubuntu?

And with this you can skip the $4 plug in the HDMI port?

Thank you!
Scott

It's an MSECORE mini PC running Ubuntu 20

FWIW re black screens, I do get them from time to time (i.e. just now). However, I've found a workaround which is to quit the NoMachine client on my macbook, restart the mac's NoMachine program, and reconnect to my Raspberry Pi. (I don't do anything to my Raspberry Pi.)  It always has returned to normal after I reconnect.