No luck with the installer as it is, but I changed the Location string to
C:\Program Files\Scrivener
and voilà! Auto-Update worked right away. Everything is updated now.

With 1.7.1.0 installed that registry value remains
C:\Program Files\Scrivener
so hopefully that pesky string won't bother us again!
Congratulations to both of you. Best Scrivener for Windows ever

Paulo