NoMachine product and version on local and remote machine (free version, Workstation, etc).
Server: RHEL 6.4 nomachine-enterprise-server-evaluation_4.0.369_2_x86_64
Client: Windows 7 64-Bit nomachine-enterprise-desktop-evaluation_4.0.369_2
Whether the problem arises connecting to a physical or a virtual display.
Physical desktop
The error is reported when the client is disconnected before getting past the session negotiation. It is likely a display problem.
It would be useful if you could provide a tar of the server logs to the support.
see attached nxserver.tar.gz