Forbidden You don't have permision to access - Additionally, a 404 not found

If you apply the right permision to the files and folders and the error still maybe this a mod_security rule issue.

Many hostings added or augmented their Apache mod_security module, adding in several new rules, which if tripped, generate a 404 permission denied.

Removed rule that says: PHP Injection attempt in URI

If you can't fix this issue, you need to contac your server administrator and request to remove the mod_security rule.