We just checked that specific Debian version just to make sure and everything works correctly. CPU usage is normal, sessions work fine.
Regarding the upgrade, we can’t be sure about what happened. Did you install the free version on top of the existing legacy free version? Please submit nxinstall.log and nxupdate.log which you can find in /usr/NX/var/log/.
After that, I recommend a complete uninstall of NoMachine and removing /usr/NX /etc/NX and /var/NX $HOME/.nx completely, then reinstalling version 7.9.2.