Ok, it looks as if the USB connector of the interface is a bit crap. I can make the computer recognize the interface by wiggling the USB cable while it's connected. The connection is dropped as easily as it is made, so not very robust for live use this one.
For no apparent reason Hardware Manager also got past the "Device is openning" / "Error Code: XHL 18" routine and launched properly, so I decided to reset to factory settings and update the firmware.
Now I've been stuck for half an hour with this:
Can I just pull the plug?