If you have protected your WordPress Admin folder (eg /wp-admin) using .htaccess authentication, you may find that when you try to access /wp-admin (or wp-login.php) and successfully enter the .htaccess username & password, the WordPress login page shows a “404” page missing error. Don’t panic – this is easily corrected. Simply add the following line as the first line in the .htaccess file in /wp-admin
ErrorDocument 401 default
Done 🙂