Per the JSON spec, JSON may only be encoded in UTF encodings, if you want to support others you need to provide your own wrapper. If you have more information that points at a bug with UTF-8 handling, ...

I got deceived by default requests behavior for auth headers. As using a password containing some non-ascii chars, the basic auth method silently encoded in latin1, which caused authentication to fail ...

It looks like the exported file is utf-16 (at least that's what gives me legible results when I open it using that encoding), but apparently RegSetValueEx wants a utf-8 encoded string?