Windows 10 host cannot install any vm guests since Virtual box 6.+

[Hammerman]

Windows 10 2004 latest upgrade. Virtual box up to date.
Ubuntu latest 20 whatever won’t install.
Neither will Mint Cinamon nor Mate.
Windows 10 Insider won’t install.
VM installations provided adequate resources. Tweaking different settings suggested in forums have done nowt.
All fail reading the “live CD” ISO near the end of installation in each case.

[scottgus1]

This sounds like Hyper-V is on. If you want to test, open a guest vbox.log and look for these lines:

{timestamp} HM: HMR3Init: Attempting fall back to NEM: VT-x is not available
{timestamp} NEM: WHvCapabilityCodeHypervisorPresent is TRUE, so this might work...

'VT-x' might be 'AMD-V'. Either way, if you see these lines, Hyper-V is enabled. Disabling it will allow your guests to install.

To get a vbox.log, right-click the guest in the main Virtualbox window's guest list, choose Show Log. Look at the far left tab's log. In the search bar at the bottom of the Show Log window, search for this text:

Attempting fall back to NEM

If you need help checking the log, save the far left tab's log, zip it, and post the zip file, using the forum's Upload Attachment tab.

[Hammerman]

First thing I (double checked as I had never installed Hyper-V) checked was to make sure it was not. I got so disgusted i deleted all the broken installations. Will try again and post.
Windows 10 Insider just installed on a Windows 7 host using VB 6.0.22 without issues.

[scottgus1]

There's lots of other things that use Hyper-V, besides 'Hyper-V', on Windows 10. Even Home level, which won't allow Hyper-V VMs, has Hyper-V-supported services. See the 64-bits tutorial.

Windows 7 doesn't have Hyper-V, so this issue wouldn't happen there.