It has been reported that a fullscreen NX Kubuntu KDE session may be suspended when it is minimized.

It is very likely that it is a bug or configuration problem in the X.org server running on the client host that leads the local X server to crash or abort.

At the current state of investigation there is no evidence for a bug in NoMachine NX.

The issue is probably related to the non-proprietary NVIDIA legacy video driver

Workaround:

Disabling the Composite Extension in Display Settings in the NX client.