Sometimes the following error message is shown when a customer starts the game:
"Can`t open video device. Try -W3D commandline option."

What could be the problem?

Sounds like they don't have enough video memory or the application is not detecting a graphics accelerator at all (ie: graphically underpowered).

In all the cases there is pretty heavy Gforce hardware used!
So there should be no problem on that end...

I don't think that it matters how heavy your 3D card is, it must be properly installed and the driver must be installed or else it wont work.

i'd suspect a driver/directX issue, too.

Just reboot your PC. This happens sometimes when a 3D Application crashed suddenly and exiting the DirectX components was not handled correctly.