Server logs report that the  '/usr/NX/etc/users.db' does not exist, even if the file is present and has correct permissions.

This happens when the /usr/NX/var/db/running/ lock files have instead wrong permissions.

As a workaround, try to change change permissions to this file:

sudo chown nx:nx /usr/NX/var/db/running/.lock.special

or uninstall NoMachine, remove the /usr/NX directory and re-install the package.