VirtualBox fails to start with VERR_NEM_VM_CREATE_FAILED

1. Open the "Turn Windows features on or off" settings (type optionalfeatures in the start menu or Win + R).
2. Untick "Windows Sandbox" and "Hyper-V" .
3. Restart windows twice.

This is a new feature in Windows 10 v1903 that uses Hyper-V on the backend (even if it's not enabled in Windows Features), which makes this feature not compatible with VirtualBox.

According to this VirtualBox forum post, there are several features which might use Microsoft Hyper-V, which is not compatible with VirtualBox:

Look in the "Control Panel" » "Programs and Features" » "Turn Windows features on or off", and make sure that the none of the following are active:

• Application Guard
• Credential Guard
• Device Guard
• <any> * Guard
• Containers
• Hyper-V
• Virtual Machine Platform
• Windows Hypervisor Platform
• Windows Sandbox
• Windows Subsystem for Linux (WSL)

If that doesn't work, enter the following command:

bcdedit /set hypervisorlaunchtype off

followed by a cold boot, i.e. shut down the computer, pull the power plug for 10 seconds, reboot.

For me, just disabling "Virtual Machine Platform" and restarting TWICE made it work.

WSL 1 will still work. I'm not so sure about WSL 2, which is currently available in the Windows Insider Program.

Finally solved the issue.

1. Turn off Hyper-v
2. Turn off Windows sandbox

by navigating to "Turn windows features on or off".

Restart the machine twice and then try to Start vm in virtualbox.

In my case, turning Windows Sandbox and Hyper-V off and restarting my PC dozens of times didn't work. I even tried turning the features back on, then back off, then restarting another dozen times.

What did work, was disabling Memory integrity from Core isolation in Windows Security, possibly because it uses virtualization features, as do Hyper-V and the Windows Sandbox.

Now I can finally start my VMs again.