I honestly do not know what might cause this error message.
Seeing how in your initial post you've said:
And in a subsequent one:
I'd have to assume that you might have tried to use the Whonix-Installer for Windows, though perhaps mixed up the guide for the Installer with the one for manual import via VBox which is why you attempted to import images as well as download a seperate Installer for VirtualBox.
At boot, there shouldn't be a Login screen, unless you are running your Gateway in terminal only mode.
Seeing, how there seem to have been some issues from the start in regards to using Whonix-Installer but still trying to import the images seperatley, may I recommend uninstalling Whonix (Control Panel-> Programs and Features-> Whonix - Whonix for Windows-> Uninstall) and installing it again, this time by only executing Whonix-Installer.exe?
Have a nice day,