Windows 10 client fails at startup with BSOD

[Dunum]

Hi
I have two Windows 10 guests which both have the latest Guest Additions installed. Having previously worked, they both now fail at start up in the same way, displaying a BSOD showing "Kernel Security Check Failure". They restart, then attempt a repair process showing "Preparing Automatic Repair" followed by "Diagnosing your PC" and eventually "Automatic Repair. Your PC did not start correctly". "Restart" and "Advanced options" buttons are offered. Clicking "Restart" starts the complete sequence over again.

A fairly old Ubuntu guest tries to start, displays the purple blank screen then hangs with a black screen and a flashing underscore cursor.

A very old Windows XP guest works fine.

I restored one of the Windows guests (complete folder) from a backup dated 25 August 2020 - same problem.

I re-installed VirtualBox - same problem.

My suspicion is that the problem is something to do with the latest Windows feature update. All worked normally before the upgrade.

Any suggestions most welcome!

Host:
Windows 10 Pro N
Version 2004
Installed 11/09/20
OS Build 19041.508
Windows Feature Experience Pack 120.2212.31.0
Intel Core i7 3820 @ 3.60GHz; 16GB RAM
VirtualBox Version 6.1.14 r140239 (Qt5.6.2)

[scottgus1]

Hyper-V is enabled and needs to be disabled. See HMR3Init: Attempting fall back to NEM (Hyper-V is active).

The new test builds might have a fix for this problem. If you try a test build and it works, report here: viewtopic.php?f=6&t=90853, and save the installer file.

[Dunum]

Solved.
Not sure what enabled Hyper-V but disabling it sorted out my problem. I'll keep an eye out for the green turtle in future!
Many thanks for your help.
Dunum