Well, turns out I'm no longer able to reproduce the issue either. I just turned DDG Privacy Essentials back on for https://cs.opensource.google and I was able to view the site just fine.
Back when it was happening (maybe ~1 year ago?), I was using the latest versions of Firefox and DDG Privacy Essentials and it ocurred even if I went directly to https://cs.opensource.google. I had confirmed back then that when I turned DDG Privacy Essentials on I got "Permission denied", but with it off I was able to view the page.
I'm on the latest verisons of Firefox and DDG Privacy Essentials now. Seems it is no longer an issue in the latest version(s).
Back when it was happening (maybe ~1 year ago?), I was using the latest versions of Firefox and DDG Privacy Essentials and it ocurred even if I went directly to https://cs.opensource.google. I had confirmed back then that when I turned DDG Privacy Essentials on I got "Permission denied", but with it off I was able to view the page.
I'm on the latest verisons of Firefox and DDG Privacy Essentials now. Seems it is no longer an issue in the latest version(s).