Hi, it is because you are using Bulletproof security plugin and hence you’ll need to manually add HMWP rule in htaccess file as explained here – http://www.lisapearlnyc.com/wp-admin/options-general.php?page=hide_my_wp&die_message=single
I tried adding the rule to htaccess file but seems like there’s some issue and site is giving 500 error now. Can you please reset htaccess file to WP default or else provide us with FTP details so that we can fix it for you.