Hello,
to diagnose your problem more information can be helpful. Firstly turn selinux and iptables on, reproduce your problem and send logs from /var/log/audit/audit.log.
After this turn off selinux, but don’t turn off iptables. To disable selinux please open as root /etc/sysconfig/selinux with text editor and change SELINUX=enforcing to SELINUX=disabled. Check if your problem exists. If it does, please send us also content of /etc/sysconfig/selinux.
If you got different error massage with/without firewall turned on please copy this error messages. Copy also /usr/NX/var/log/nxserver.log.
Please let us know which protocol are you using (NX/SSH), and which version of NoMachine you got installed on server/client.
regards