Could you please specify the exact version of RHEL are you using?
Do you confirm that just installing version 4.2.17 is enough to create the problem? Does the problem show up in the console even if you never used NoMachine for the remote access?
Does the problem occur after you stopped the server (/usr/NX/bin/nxserver –shutdown)?
Is it possible for you reproducing the problem and sending the server logs and the output of ps -ef –forest?
You can find instructions to gather server side logs here: https://www.nomachine.com/AR07K00677