I’m witnessing the same behavior on Arch Linux with Intel graphics, Gnome 42.4 on X11 , physical display, NoMachine 8.0.168 installed from the AUR:
The first connection after a server restart works fine, but any subsequent connection results in a white screen. I tried various clients, which clearly points to a server problem.
Disabling hardware encoding on the server solves the issue.
I also tried a fresh NoMachine DEB install on a Ubuntu live CD on the same machine and the same configuration (i.e., hardware encoding enabled). In that case, multiple successive connections work fine.
So, I would say it’s a server configuration issue specific to Arch and hardware encoding with Intel graphics.