On the KMymoney mailing list Brendon Coupe mentioned that the x86_64 installer uses the incorrect default installation path "C:\Program Files (x86)" instead of the correct one, which would be "C:\Program Files".
Fixed in installer and will be available with released version 4.8.2 or an updated build of 4.8.1.1 available at https://build.opensuse.org/package/show/windows:mingw:win64/mingw64-kmymoney-installer