This error is caused by the following conditions:
- Directory listing (listing of the files in the directory) has been disabled
- There is no default page (index.html, index.htm, etc) to show
If you have a unique default page such as default.php or home.html that perhaps isn't on the list of default page, you need to tell the web server to load it as the default page.
This is done through the control panel by going to Web Options -> Directory Indexes.