What are you trying to accomplish ? FreeNX doesn't do any authentication by itself by default; Thats all done by PAM (unless you disabled PAM support). EDIT: Just realized somethin... [by KermitDaFragger]
since some days I try to get freenx (CentOS package 0.7.3) running under CentOS 6.3. It is like found on many websites: First login is successfull, after that all login attempts fail with the negotiation error.
A simple ssh with the same username to the server is successful.