403 Forbidden Error means that your request to accessing a file or resource is being denied by the web server. Mostly this error is caused due to mis-configured permissions or .htaccess file, other times it could be caused by our mod_security rules as well.
Check the error log available from cPanel to help you determine the cause of the error.
If your error log shows "client denied by server configuration", this usually means that your script/file/folder has invalid permissions or the visitor is trying to look at hidden files such as .htaccess file (any file name that starts with a dot. are not accessible on the web)
If the error is occurring due to permissions, make sure all Directories and folders are set to 755 permissions. Scripts within the cgi-bin folder should be 755 permission. Images, media, and text files like HTML should be 644. PHP scripts should be 644 as well.
Make sure all your mod_rewrite values are correct, there are no typos or any misconfiguration.
Other possibilities could include blocked IP, make sure you are not blocking the IP from cPanel's IP Deny Manager. You could have also changed the Index Manager to No Indexing from cPanel. This will lead to a 403 error if there is no correct index page to load.
If the error is being caused by mod_security, simply submit a support ticket for further help.