Now, if we want to send a signal (either 0, 1, or 2) via Inquisit, the machine doesn't respond (only when we actually close the serial port monitor, which in itself is quite strange as well...). Also when we program a test-script, the machine doesn't respond to the signal we send.
Not sure what you actually mean to say here, but if you cannot trigger the device via Inquisit's Serial Port Monitor (Tools -> Serial Port Monitor...), you probably won't be able to trigger it via syntax either. While there are conceptual similarities, the USB standard is substantially different from the RS-232 serial communication standard.
Might it be due to the fact that the item of a port element is required to have 8 digits?
<port> elements in Inquisit do *not* send 8 digit *numbers* or other arbitrary strings. They only send 8-bit *binary* values, your device might or might not be able to make sense of. FWIW, of course
------------------------
Decimal -> Binary
Base 10 -> Base 2
------------------------
0 -> 00000000
1 -> 00000001
2 -> 00000010
------------------------
(bit order might have to be reversed). If your device expects ASCII data, though, I think you're out of luck.
Regards,
~Dave