Windows 10 boot loop and error 0xC000021A

Toshiba P50 laptop, 16G RAM, 1Tb disk, Windows 10 home

A couple of weeks ago my laptop reported this error at boot: Your PC ran into a problem and needs to restart and error 0xC000021A. I could get it to start by disabling driver signature enforcement, but this wasn't reliable/consistent. I would then make sure I hibernated instead of shutting down, but eventually it would somehow restart in the dead of night.

After booting to Hiren's Mini-XP I could run CHKDSK and SFC but these found nothing wrong. Memory test and low-level disk scan are OK. Eventually I gave up and restored a Macrium backup made a few weeks ago, and things seemed back to normal. I could shut the PC down and start up again without incident.

However tonight I received the "I'm going to restart and update your PC whether you like it or not and there's nothing you can do to stop me" message, so I thought I might as well at least watch it and see if it successfully restarts and updates. That was at midnight, it's now 1:45 am and it's onto it's fourth or fifth attempt at booting and currently showing we couldn't complete the updates. Undoing changes

** POSSIBLY RELATED **

For some months now the laptop has been trying to install a Windows Update 1607 and failing - it would report 'undoing the changes' at boot time. This was more an annoyance than anything, as it meant the PC would restart (and fail to install the update) frequently overnight. Looking in the installation history, I see it finally succeeded tonight, just when the troubles started again.

Most probably caused due to Microsoft drivers.

Boot in safe mode (or from restore point if you can't stop the update at all).

Go to start menu and search for "device installation". Select "no".

After that you'll need to install the manufacturer drivers (google "toshiba p50 drivers"; make sure to select/specify the correct model and OS on the top right corner). Chipset, Proset, Wireless and Display Driver seems most important to me.