Problem:

On Centos 6.3, I installed FreeNX and work with it like a week… after restart Centos, I tried to log in to FreeNX and the session authenticates and the black window with the big “!M” appears for a second or two. Immediately afterward, the session is terminated and the window closes without any errors written to the logs.

 

After lots of searching in the internet :http://ubuntuforums.org/archive/index.php/t-1159771.html:

I found one solution that help me:

just delete the files: .Xauthority and .ICEauthority from the user directory, login to the machine using Putty.

and the NX back to life!