I know this is a very old topic however I just dusted off my 495Pro now that I have solid internet.
I attempted the flash several times but I keep getting the error that there is an error in the zip file and it can't flash. Consequently I no longer have a system file and I have to reboot to recovery and flash the 5.1 firmware again which works wonderfully. Both downloaded onto the same USB from the same computer from the same source. Any idea why?