The HTTP 403 Forbidden response status code indicates that the server understands the request but refuses to authorize it.
Although web server security is important, sometimes it can go overboard and prevent things from working correctly. Here are some common causes of problems for Simple File List:
Web Host Security
Plugins such as SG Security, Admin Tools and the like can prevent execution of PHP files within the plugin directory. File Access Manager needs to access its own files to check if users can access items.
Check the plugin settings to see if you can disable this feature.
Folders Created with FTP
If you create folders outside of the plugin, using FTP or another means, sometimes the permissions can be wrong. Check the existing settings for the default file list (/wp-content/uploads/simple-file-list/) and match them.