Thank you for trying to fix this!
What is VirtualBox version?
Should be 6.1.6 or 6.1.8. Not older.
Can you boot into Recovery Mode?
You could try to change kernel boot parameters. In grub boot menu press E to edit. Use arrow keys.
Delete
spectre_v2=on spec_store_bypass_disable=on tsx=off tsx_async_abort=full,nosmt mds=full,nosmt l1tf=full,force nosmt=force kvm.nx_huge_pages=force random.trust_cpu=off intel_iommu=on amd_iommu=on efi=disable_early_pci_dma slab_nomerge slub_debug=FZP mce=0 pti=on vsyscall=none extra_latent_entropy
(Also others. Not have full list handy.)
boot using
ctrl + x
Search engines will have instructions too for how to edit kernel parameters in grub boot menu.
Can you boot that Whonix VM (the workstation) from that Debian Live DVD that you downloaded (which works for you)? Please try to insert it into the workstation and boot from virtual DVD (Debian Live) rather than virtual hard drive (Whonix). (This is to find out if Whonix’s VirtualBox host settings are the cause.)
You could try to re-install VirtualBox.
You could try on a different host (computer).
You could try on a different host operating system such as Debian.
You could also report a bug against upstream VirtualBox [archive], VirtualBox bugtracker [archive] / VirtualBox forums [archive], but it needs to be a good bug report. Otherwise the report will not lead to a solution. They’ll require VirtualbBox logs. Instructions:
https://forums.virtualbox.org/viewtopic.php?f=3&t=91874#p442720
Please post related VirtualBox forums threads here too so I can jump in.
You could try a Serial Console.