VBox crashes after host update to 20H2

[Hiro Protagonist]

On Friday my laptop was updated to 20H2, and today I discovered that VirtualBox no longer works. On trying to start any of my VMs (32-bit Unbuntu, 64-bit Ubuntu or 64-bit Win 10) I get this message:
"The instruction at 0x00007FF873F7C2D3 referenced memory at 0x000000000B10FFFF. The memory could not be read."
I tried updating to the latest version, and this makes no difference.
Has anyone else had this problem? What can I do?
I've attached the log of the failed startup from the 32-bit Ubuntu.

[scottgus1]

Please right-click the guest in the main Virtualbox window's guest list, choose Show Log.

Search the far left tab's log for this text:

Attempting fall back to NEM

If you find it, Hyper-V is enabled. See HMR3Init: Attempting fall back to NEM (Hyper-V is active).

If you don't find that text, save the far left tab's log, zip the log file, and post the zip file, using the forum's Upload Attachment tab.

[Hiro Protagonist]

The line "Attempting fall back to NEM: VT-x is not available" is present. Task Manager tell me that VT-x is enabled. Checking "Windows features" tells me that Hyper-V is disabled...

[Hiro Protagonist]

OK, "Microsoft Defender Application Guard" had been turned on by the update. Turning it off and rebooting fixed the problem. Boy, that Microsoft Defender Application Guard knows how to defend against people trying to run their applications!

[scottgus1]

that Microsoft Defender Application Guard knows how to defend against people trying to run their applications!

Glad you're up and running!

[mpack]

Boy, that Microsoft Defender Application Guard knows how to defend against people trying to run their applications!

I see their reasoning. In fact the safest computer is one that cannot be switched on!

[Hiro Protagonist]

Glad you're up and running!

Many thanks for your incredibly swift help!