Started having the same trouble today at around 17:00. It fails on two accounts on my desktop but not my laptop. Both the laptop and desktop has one standard and one 4k display. The start button on the ribbon is the tailored one from start 10. All attempts to go to win 8.1 and start10 format fails.
Re-installed start10. Still no joy. Using:
Version 10.0.14393 Build 14393