Problem

If changed your default umask from 022 to something more strict and get Error 403 for your website after putting your _site’s files via ftp:

Your file permissions are not what the need to be for your webserver.

Solution

Remove the _site directory:

rm -r _site

Build your site with:

(umask 022 && jekyll build)

Using jekyll serve here worked not as anticipated, so I’ll stick with removing _site after using serve.