Using c# to connect to USB device fails

Hi all,
I’ve got a small c# program that uses LibUsbDotNet.dll to create a connection to a USB device (not HID). It works fine.
I’ve imported the program into Unity, added the DLL.
There seems to be no errors, but the debug info shows that the device is not found. If I try (same machine) to connect using just the c# program, it does work.

Anyone any idea how to solve this kind of connection problems?

Thanks ever so much,
Marco

Bump.