Pretty late to the party but here is how I fixed it for myself today.
Go to settings for your repository: You can find Settings tab in your repo page.
Scroll down to GitHub Pages section on the settings page.
In the panel, you will have a Source info that states : 'Your GitHub Pages site is currently being built from the gh-pages branch'.
However, in my cases, all code was in master branch. So I selected the branch from dropdown as master and in merely a minute, it was published successfully.
