I have the same issue. Once the new plugin is activated and the Hide Backend feature saved (with the same setting as used previously) all I get is 404 Errors on any login page wp-login.php, wp-admin and the secure url set previously. Fortunately, our site is hosted on WPEngine so I’m able to restore to a previous point in time but most of our clients using this do not have that luxury. Please advise when this will be resolved so we can get client sites updated accordingly.