Hello,
I had the same problem searching around for days.
The cause was NOT a installed / uninstalled Virtualbox BUT a portable one on the filesystem :-(
In my case i opened C:$WINDOWS.~BT\Sources\Panther{RANDOMSTRING}_APPRAISER_HumanReadable.xml
and search for 'T_ANY_FMC_BlockingApplication" Value="TRUE"'
which found:
<Property Name="LowerCaseLongPathUnexpanded" Value="%programfiles(x86)%\linuxlive usb creator\tools\virtualbox\portable-virtualbox\app64\virtualbox.exe" Ordinal="1" />
Removing this files let me install 20H2.