@lug42 Is your system clock accurate?
I had that problem installing on my Chromebook after swapping the hard drive, and the reason is that for some reason the system clock runs at hyperspeed between when you plug the battery in and plug it in to mains for the first time. By the time I finished putting it back together, the silly thing thought it was the year 2580 and it was causing a crypto failure accessing the mirrors. When I set the date manually (it was too far out even for Internet time to work), and then updated with Internet time the error went away
I have no idea if this is what you’re experiencing, but it only takes a minute to check.