This has been resolved by Microsoft and Oracle. Ensure that your version of Windows is Windows 10 1909 or later and install Virtual Box 6.12 or later which will resolve the problem.
Due to a security feature in Windows 10, Oracle's VirtualBox will now blue screen a Windows 10 PC whenever you try start a VM in VirtualBox. The bluescreen error is vt-x is not available.
There are two possible ways to resolve the error until Microsoft comes up with a permanent fix.
Install the Device Guard and Credential Guard hardware readiness tool available from Microsoft at: Device Guard and Credential Guard hardware readiness tool. However, this fixs only works for a month until the monthly security patches are released in which case you have to run the tool again.
The other option is to uninstall the Hyper-V Windows Feature from the Control Panel.
The VirtualBox team is aware of the problem and has a ticket open for it Ticket 16801 However, the ticket has been open since May 2018 and the general concensus is that Windows can only have one Hypervisor running at a time and the problem should be resolved by Microsoft and not Oracle/VirtualBox team.
Last updated 2020/02/12