The infamous Apache2 “You don’t have permission to access / on this server” error started appearing after I updated my Mint/MATE system (Ubuntu) to version 17 (14.04). It is fixed by editing /etc/apache2/apache2.conf. Details in this article.
“You don’t have permission to access / on this server” in Ubuntu / Apache2 is due to not having the file system root (not the web site root) executable to Apache2. Every directory from the web site root up to the file system root must be executable by Apache2.