Cannot Install BitDefender Free - 'Installer Initialization Failed'

pkr

Before buying the family pack I wanted to try the free version, however, it will not install, I just end up with 'Installer Initialization' failed' message. I previously used Avast, which I uninstalled with the proper uninstaller tool. I have also run the Bitdefender Free uninstaller tool (as advised on support webchat) and then tried to reinstall, but cannot get this to work. I only ever get the error message above.
I'm on W10 Home. This is my last ditch attempt before I give up on BitDefender entirely, so I'd really appreciate any help with this one.