It took several attempts. Apparently the installer doesn't like external drives connected, even though it's installing to C:

It still failed with the same message, but after clicking Retry, the program actually installed. When first running it, the program corrected some missing config files and started.

The install must be run as administrator, even though you may be an admin account, and the program must be run as administrator as well.

Seems that this is something that could be fixed!