Pixhawk 4 PX4 BL FMU hangs with Linux (netrunner arch)

Hi
I am using APM planner 2.0.26 and 2.0.27

While trying to flash new firmware, I follow the on screen instruction, the Pixhawk with cube (PX4 BLFMU) is found on /dev/ttyACM0 - and i have udev rules set so that all users can read & write the port.

But the flash hangs with the log ::

[20190914 22:31:51.201 DEBUG] - flashButtonClicked
[20190914 22:31:52.559 DEBUG] - Go download: “http://firmware.ardupilot.org/Rover/stable/PX4/APMrover2-v2.px4
[20190914 22:31:53.462 DEBUG] - Download finished, flashing firmware
[20190914 22:31:53.464 DEBUG] - Temp file to flash is: “/tmp/APM Planner.jNhDsH”
[20190914 22:31:53.476 DEBUG] - PortName : “ttyS1” Description : “”
[20190914 22:31:53.477 DEBUG] - Manufacturer: “”
[20190914 22:31:53.478 DEBUG] - PortName : “ttyACM0” Description : “PX4 BL FMU v2.x”
[20190914 22:31:53.478 DEBUG] - Manufacturer: “3D Robotics”
[20190914 22:31:53.479 DEBUG] - PortName : “ttyS0” Description : “”
[20190914 22:31:53.479 DEBUG] - Manufacturer: “”
[20190914 22:31:53.513 INFO ] - Firmware size: 900120 expected 900120 bytes
[20190914 22:31:59.668 INFO ] - Port found! “ttyACM0”
[20190914 22:31:59.732 INFO ] - Device Info read complete
[20190914 22:31:59.735 INFO ] - SN read complete
[20190914 22:31:59.737 INFO ] - Requesting erase
[20190914 22:32:00.451 INFO ] - Erase complete
[20190914 22:32:00.452 INFO ] - Flash requested, flashing firmware
[20190914 22:32:00.453 INFO ] - flashing: 0 / 900120
[20190914 22:32:00.546 INFO ] - flashing: 3000 / 900120
[20190914 22:32:00.640 INFO ] - flashing: 6000 / 900120
[20190914 22:32:00.735 INFO ] - flashing: 9000 / 900120
[20190914 22:32:00.820 INFO ] - flashing: 12000 / 900120
[20190914 22:32:00.901 INFO ] - flashing: 15000 / 900120
[20190914 22:32:01.267 INFO ] - Bad sync return: “12” “13”
[20190914 22:32:05.101 INFO ] - Bad sync return: “12” “13”
[20190914 22:32:08.102 INFO ] - Bad sync return: “12” “13”

Please help, thank you