Update MP to latest Beta. Help screen button on the bottom.
Good day, your small issue can be related to driver issue or forcing the bootloader you have corrupted it.
If you need i can prepare a file so you must restore it completely with a new one…after this you have only flash the firmware and after few settings you are ready to fly
I finally solved the problem and managed to flash arducopter 4.0.3 to my Pixhawk.
Reason is : in QGroundCOntrol, when updating firmware/bootloader, I did not change the default flight stack, leaving it as PX4 while I wanted to flash arducopter, which I did after updating bootloader.
Thank you for yr support so far.
Frankie