NoMachine require associating local IP address 127.0.0.1 with localhost hostname.
Please check if you can (temporarily) set the value for localhost to ‘127.0.0.1’ in ‘/etc/hosts’ file on the troublesome server.
Then after restart of nxserver it should be possible to establish sessions.
If the issue persists please gather debug information following the instructions here:
Send them to issues[at]nomachine[dot]com with your ‘/etc/hosts’ file.