That is a good suggestion to check if the user is in the group plugdev.
Actually I was, the problem was how difficult it is to hold the reset/boot button pressed. After 3 attempts with two hands (essential) and with the RAE open so that we had sensory feedback we succeeded. The LED2 has to be a constant blue when you connect the USB-side. You can check if you are succesfull because the device becomes visible with "lsusb | grep Movidius". If you get a response with Bus and Device the RAE is in recovery mode.
The flashing started but crashed after updating only part of the images that should be written. After a third flash attempt we got the output:
-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 created GPT in 0.0160s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed data in 26.9830s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed boot_a in 16.9080s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed boot_b in 16.7980s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed system_a in 208.3450s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed system_b in 311.7550s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed syshash_a in 5.6530s
3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5 flashed syshash_b in 5.5260s
OS flashing done for device 3-4 Intel Keembay USB(in OS recovery mode): 58927016838860C5. Please shutdown the system, set the boot switch to normal and reboot the system
Successfully flashed device with MX ID: 58927016838860C5 on USB port: 3-4
After the reboot the device is not visible with lsusb, but dmesg indicates that usb0 was renamed to a USB ethernet device and we could ssh to the default ip 192.168.197.55.
Connected the device to our wifi and was looking in RobotHub for the default Luxonis app when the RAE disconnected. It flashes RAE on its little screen and goes black again. Seems to be rebooting. No longer reachable with ethernet, nor ssh nor RobotHub. Could be again a battery failure, looking if charging helps.