The error StdErr you get is when the port (USB ZWave key) is not connect.
If you get many time that error, that means that USB is disconnected, or perhaps if you use Raspberry, the main supply is not strong enough. If it's 1 Amp it is not suffisant prbably. Try a main supply of 1.2 ou 2 Amps will be a good test.
Sure if ZWave controller is deisconnected many time, you get an erratic result. You must never get that error of StdErr on ZWave controller.