HSTS is just an HTTP header which tells browsers "next time you're going to access this site don't bother using HTTP, go straight to HTTPS". There is nothing it can break. I have to insist that since your URLs are already HTTPS and you are getting a 403 Forbidden this means that a .htaccess file anywhere in the path of this file is preventing direct web access to our index.php file. This is usually something you do through a security plugin or by modifying your site's .htaccess file manually. Since you added the HSTS header I understand that you edited your .htaccess. Please double check what other changes you made.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!