I ran into this problem, and my solution was moreso that www-data didn't own the proper folders, and instead I set it for one of the users to own it. (I was trying to do a bit of fancy, but erroneous trickery to get ftp to play nicely.)
After running:
chown -R www-data:www-data /var/www/html
The machine started serving data again. You can see who currently owns the folder by means of
ls -l /var/www/html
