There's a random VirtualBox 6.x crash issue in Windows 10 2004 when using latest GPU drivers. The error looks like "The instruction 0x00007FFD2E6AF43C (or some random hex) referenced memory at 0x0000000000000000". There are 2 ways to fix it.
The first way is to change the Virtual GPU to vmsvga. This will also get rid of 3D acceleration, so if you need 3D you will want to do the other way instead.
Double click the VM
Go to Display
Change the Virtual GPU to vmsvga.
The second way is to install VirtualBox 5.2.
Uninstall the Extension Pack from the settings (if you have it)
Close VirtualBox
Uninstall VirtualBox in Programs
Reboot your PC
Install VirtualBox 5.2
Optionally install the Extension Pack
After doing this, everything should work as expected.
A third way would obviously be waiting for the developers to fix the problem.