During the search for local sessions on Linux platforms, the memory usage of the nxserver --daemon process grows.

Restarting nxserver allows to free memory: 

sudo /etc/NX/nxserver --restart

As a  workaround for servers which support virtual desktops, remove 'physical-desktop' from the AvailableSessionTypes key in the server configuration file (/usr/NX/etc/server.cfg). This workaround cannot be applied to servers supporting only physical sessions, like NoMachine free or the Enterprise Desktop.