Hi,
you’re right, by default Magboltz tries to guess a suitable upper limit of the electron energy range. This seems to fail in your case (not sure why, will take a look, but it might be because of the low pressure).
As a quick fix, you could switch off the automatic determination of the energy range, adding a line
gas.EnableAutoEnergyLimit(false);
and set the upper energy limit (in eV) explicitly, e. g.
gas.SetMaxElectronEnergy(100.);