After the above failure, I saw no devices on the new hub. So I tried updating the hubitat version on the new hub (to 2.3.6.146), and then suddenly all of my devices showed up. But none of them work. I tried turning on and off zigbee bulbs, z-wave switches...nothing happens. I tried the restore from migration backup again, and same "internal error". Plugged my old C-5 hub back in and thankfully everything still works. No clue what to do at this point.
We've had a few incidentes reported recently with migration failing from C5 to C8 and we are currently investigating. Please send me a private message along with your hub id.