I guess, for security reason, the server will not allow directory indexing by default, which means if your folder does not have an index.html it will not show any webpage and give the permission error. You can google 'htaccess' if you want more details.
It's typical that if a directory does not have a home or index file then nothing shows. You need to know the name of a file in the directory in order to access it. Often times when a file is moved a link to that file is made for the user or an index.html file is updated with the addition of the filename. Bob