A little late on this but PAY ATTENTION..
I had the exact same thing thing with the constant "update flash" message.
The power on and holding ANY of the arrows did NOT work for me, neither different usb or pc.
I am not 100% which one of the following steps worked but something worked.
my fixing story includes:
Have the guitar jack plugged in the pod
Power on the pod holding down the "MOVE" button.
It should take you to the regular kinda screen
try to create and save a random sound..nothing special just put a random amp and save.
after that try unplugging and plugging the power holding the left arrow of the 4 way nav pad. that should global reset.
then try the same unplug and plug thing holding the right arrow of the 4 way nav pad and try the pedal calibration (https://www.sweetwater.com/sweetcare/articles/line-6-pod-hd500x-tips-pedal-calibration/)
now the most important step.. Once again turn the pod on holding the move button in order to be taken into the regular pod screen.
use A DIFFERENT USB to MIDI cable and connect the pod to your pc.
open monkey app and try to reinstall the update locally or normally, it didn't matter for me.
after the update finishes, follow the normal process of unplugging waiting almost 5 seconds and plug the pod into power again..
that worked for me. I came to the conclusion that it was a matter of CORRUPTED or NOT completely compatible usb to midi cable.
Try to update the pod with another cable and if you can not get the pod to open pass the ""update flash"" screen, then try the trick of power on the pod by holding down the NAV button AND having the guitar jack plugged in the pod.
hope that works for you too..
