This message is generally caused because either

• The underlying file system permissions do not allow the User/Group under which Apache is running to access the necessary files; or
• The Apache configuration has some access restrictions in place which forbid access to the files.

You can determine which case applies to your situation by checking the error log.

In the case where file system permission are at fault, remember that not only must the directory and files in question be readable, but also all parent directories must be at least searchable by the web server in order for the content to be accessible.