Virtual machine fails to start after updating to 2004

[pstrg]

Windows 10 Pro machine updated from 1909 to 2004 yesterday.
Today, trying to start a Debian VM which was working normally with 1909, got error: E_FAIL (0x80004005).

Just in case, checked Windows features:
Hyper-V was OFF - kept it off.
Virtual Machine Platform was OFF

Tried to set Virtual Machine Platform setting to ON, restarted twice to no avail.

Searched for a similar situation on the net, found none.
What may be the problem?

[scottgus1]

Start the guest from full normal shutdown, not save-state. Run until you see the problem happen, then shut down the guest from within the guest OS if possible. If not possible, close the Virtualbox window for the guest with the Power Off option set.

Right-click the guest in the main Virtualbox window's guest list, choose Show Log. Save the far left tab's log, zip it, and post the zip file, using the forum's Upload Attachment tab.

[pstrg]

Thank you - that's quick!
As per instructions (hope).

[scottgus1]

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

[pstrg]

No, believe me - it's not; the checkbox is not marked on Windows feature.
I'm aware of the incompatibility...
It was OFF on 1909 and continued OFF after the update to 2004.

[mpack]

We'd rather believe the log. VirtualBox isn't mistaken about this kind of thing, plus we've already had this exact conversation about 1000 times.

00:00:02.727756 HM: HMR3Init: Attempting fall back to NEM: AMD-V is not available
00:00:02.790171 NEM: WHvCapabilityCodeHypervisorPresent is TRUE, so this might work...
00:00:02.790183 NEM: WHvCapabilityCodeExtendedVmExits = 0x00000000000003ff

Read the relevant part of the FAQ: viewtopic.php?f=1&t=62339#p452019.

[pstrg]

@scottgus1, @mpack,
First and foremost, I was not contesting the answer - it would be quite illogical to seek help from those who know more and deny their advice...

The article cited in the last post brought the answer; according to it, none of the list below should be enabled (not only Hyper-V) - strange as it may be, some of them implicitly enables Hyper-V.
In my case, Windows Sandbox was enabled; after I disabled it, things were normal again.
Thank you very much for solving the case; amazed at the promptness of the replies.

Application Guard
Credential Guard (see https://www.microsoft.com/en-us/downloa ... x?id=53337)
Device Guard (see https://www.microsoft.com/en-us/downloa ... x?id=53337)
<any> * Guard
Containers
Hyper-V
Virtual Machine Platform
Windows Hypervisor Platform
Windows Sandbox
Windows Server Containers
Windows Subsystem for Linux 2 (WSL2) (WSL1 does not enable Hyper-V)

[scottgus1]

Great! Glad you're up and running.

Typically speaking, the 'bcdedit' command, when properly run, is the silver bullet that lays out Hyper-V et al.