thank you for your reply. I did not know about GDM at the time.
As I needed to resolve this quickly, I removed the NVIDIA (originally installed from first principles) drivers and re-enabled nouveau. After this was done, NoMachine worked as expected in all aspects. I then re-installed NVIDIA drivers using the CENTOS 8 preferred method using kmod-nvidia from elrepo. This process went smoothly and when rebooted, NoMachine server was again functioning faultlessly.
Backup/restores have destroyed the logs associated with the original problem. However, I have to conclude that the problems from my earlier post were due to a messy installation of NVIDIA drivers and nothing to do with NoMachine, which is a perfect product for my purposes.
Thank you for explaining the place of VirtualGL. I will remove this.