dalto You don't need to submit evidence.
Thanks, but now I'm determined. And really curious.
The one thing that annoys me about VBox is that "Solus" isn't a selection for "type of Linux" when it comes to creating a VM. Yet, I've created dozens of Solus VM's in the past. I just haven't done that for a long time, as I've had no need to ... all my machines (currently three laptops and two desktops) run Solus, and I've only needed to make a couple of Windows 10 VMs to support my Ring doorbell and Nook reader. Those are so primitive they don't have Linux versions available, just Windows and Android.
When I made the VM that worked earlier today, I'd chosen "Other Linux(64-bit)" a the type of VM to create. That worked, but as you pointed out, it was a BIOS machine. When I tried that with "Enable EFI" checked, I couldn't start the VM. Then I tried selecting "Ubuntu (64-bit) as the Linux type, and that failed as well. There are a LOT more choices.
I guess I need to pick out a Linux type that supports UEFI, but doing that by experimentation is going to be a long process. I'm determined, though. And still curious. I'm just afraid it won't be quick.