When the physical session is running under Wayland, after a couple of days the 'nxserver --daemon' process begins to report errors about 'Too many open files', e.g.:

NXCat: ERROR! Cannot open '/run/systemd/sessions/169': Too many open files.

Users are no longer able to connect to that machine. Connections to the physical display fail, as well as starting of virtual desktops or custom sessions.

Possible workarounds:

If connections to the physical desktop are not needed, edit /usr/NX/etc/server.cfg and remove 'physical-desktop' from AvailableSessionTypes key.

Then restart the NoMachine server:
sudo /etc/NX/nxserver --restart
 

If it's not possible to disable connections to the physical desktop, disable Wayland in the display manager configuration.

E.g. in case of GNOME desktop, add the following key to /etc/gdm/custom.conf:
WaylandEnable=false

and restart gdm:
systemctl restart gdm

Then restart NoMachine server
sudo /etc/NX/nxserver --restart