Server side logs will be helpful.
Please follow the instructions here and either attach them to your reply or forward them to issues[at]nomachine.com.
https://www.nomachine.com/AR07K00677
Additionally please:
– provide a output from ‘ps -ef’ command running as root;
– tell us a exact version of RHEL;
– tell us if it’s a headless server or you have a desktop (Gnome, KDE) actually running;