Hi,
Did you enable the virtualisation options for the VM? Eg. under settings -> Processors & Memory -> Advanced options.
If any of the options there is checked then try to remove those checks.
You can also try to change with or without side channel mitigations (settings -> Advanced -> setting near the bottom).
PS: nothing to do with Big Sur? It's using apple's hypervisor now, so there's a reasonable likelyhood that it does have at least a partial reason there.
re. Windows 10, not sure exactly which build you are on, there are some issues with running VMs and the most recent insider previews.
--
Wil
