virtualbox.org

Issue: display defects, visible mainly on Windows 10 system menu and main system bar showing strange transparency and rendering issues. Please see attached screenshot. Options from Windows system menu doesn't react on clicking. The whole system menu is unusable. System has to be switched off externally.
System is also booting much slower and feels sluggish. Some applications are usable, like Firefox.

VirtualBox version: 6.1.14 (latest) but also observed with older versions 6.0.x
Host: Windows 10
Guest: Windows 10
VirtualBox both host and guest extensions installed, same version as above.
Setting -> Graphics Controller: VBoxSVGA
If I choose anything else, like VBoxVGA or VMSVGA or even None, system complaints "Invalid settings detected" and prevents starting. So I'm stuck with VBoxSVGA.
Issues appears only when 3D Acceleration is on

When 3D Acceleration is on, guest VM runs fine and seems to be usable.

Is there anything I can do to fix this?

Reduce guest processor count to 2. Windows 10 runs fine on 2, but until you have more cores of data to shove through the processor, your guest may run slower with 6 cores instead of 2, due to extra scheduling oversight on the host OS.

Try disabling Settings > Personalization > Colors > Transparency Effects in your Windows guest.