This issue looks the same as another that recently popped up:
But your additional error info is giving me a hunch. A question for you - is your repository’s primary branch named “master” or “main”? Based on the error coming from the Git Gateway and /branches/master in conjunction with GitHub changing the default primary branch name from ‘master’ to ‘main’, I’m wondering if Netlify CMS is just failing to find a ‘master’ branch
