I tried with the multimeter the continuity between the end of a USB cable connected to the Duet and several points in the Duet's PCB around the USB connector (R1, R2, ...). I have found continuity between the four cables of the USB connector and the points that I have mentioned. The cables have been tested individually.
I think this discards a bad welding of the USB connector to the PCB.
Regarding the USB pins of the microprocessor and the resistors, I do not know how to measure it. I have taken a picture of the microprocessor pins and I have not seen anything strange, but that does not rule out anything.
Thank you again