Dear reza,
our system admin followed your suggestions:
root:~ # cat /usr/NX/etc/node.cfg | grep DefaultXSession
# DefaultXSession /etc/X11/Xsession
DefaultXSession /etc/X11/xinit/Xsession default
root:~ # find /etc -name Xsession -exec ls -l {} \;
-rwxr-xr-x 1 root root 3563 Aug 17 2010 /etc/X11/xinit/Xsession
lrwxrwxrwx 1 root root 21 Jul 16 2012 /etc/gdm/Xsession -> ../X11/xinit/Xsession
however, I still get the same error message when trying to use custom session.
any other thoughts?
Thanks!