I've had related problems to this, in my case it was because I was using an old version of macOS and Xcode, that didn't have the newer Apple Worldwide Developer Relations certificate installed. You can install it yourself by downloading it from https://www.apple.com/certificateauthority/.

I want to autogenerate some documentation for some cross-language bindings I'm making, and so it would be nice to have access to the documentation in raw form.