The following warning is printed in the server logs of NoMachine free version:
7:42 341.046 NXSERVER WARNING! Cannot open: /usr/NX/etc/htd.cfg.
This warning is harmless and can be safely ignored. The htd.cfg file is the configuration file of nxhtd, the minimal Apache web server used to deploy sessions by the web. Such program is not included in the NoMachine free version.