bricked APM 2 5

APM 2.5 can't communicate anymore with mission planner via USB nor via 3DR radio. The video shows these symptoms and the error produced when trying to flash a new firmware (and having tried unsuccesfully to flash the MEGA2560 bootloader beforehand via usbasp-ISP programmer). The ArduPPM encoder firmware has been reflashed succesfully but this did not solve the usb comminication issue.