If you have protected your WordPress Admin folder (eg /wp-admin) using .htaccess authentication, you may find that your site design breaks or some features don’t work.
This can be due to Ajax functionality failing because of the .htaccess authentication is blocking access to the Ajax functionality required by the web site.
Try this – simply add the following lines to top of the the .htaccess file in /wp-admin
<Files admin-ajax.php>
Order allow,deny
Allow from all
Satisfy any
</Files>
Also, 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
Hope this helps. 🙂