What I usually do is to start the upload from the Arduino IDE, and when the log says:
I quickly connect the USB cable from the computer onto the Olimex board. Normally the DFU device is enumerated and the upload starts.Searching for DFU device [1EAF:0003]...
Assuming the board is in perpetual bootloader mode and continuing to attempt dfu programming...
Until yesterday where maybe the USB connector was not fully inserted, or whatever reason. The fact is the upload failed, and since then the board fails to enumerate the DFU device. On my computer, a faulty device is shown : "unknown USB device (device descriptor request failed)."
Long story short, my board is bricked.
However, the computer works perfectly with another board; in addition, the faulty board still operates normally with the last sketch uploaded to it.
So it seems that it is only the USB function that fails on this board.
I am wondering whether the bootstrap loader could have been damaged in some way by an aborted programming sequence? In this case I might be able to recover by flashing the BSL again using ST-Link for example? Or is it rather the physical USB interface (it is directly connected to two pins of the microcontroller) that has been damaged?