Cannot open DFU device error

You can check what driver is actually in use.

To do that you would:

  1. connect the board to the computer
  2. (eject safely if needed)
  3. hit upload, wait for it to fail
  4. find the device in “devices and printers” or the device manager. (It will be called “STM32 BOOTLOADER”)
  5. right-click and select properties, then “drivers”
  6. The driver should be WinUSB if I remember correctly. If it is something else, then that something else is probably what is interfering.