Server Error 404 Not Found in admin-ajax.php

Getting a 404 on actual files like admin-ajax.php (as opposed to WP “pretty” permalinks where no actual file exists) is usually a server level issue and is external to WP. WP is not involved in fulfilling requests to actual files.

Via FTP, first confirm the file actually exists. Once common cause for this is the file has incorrect permissions. Usually it should be 644, but sometimes 664 is necessary. Other causes are .htaccess directives that prevent access or security measures like modSecurity.

A common mistake is to block access to /wp-admin/ as s security measure. This is OK, but access to admin-ajax.php would then need to be an exception. If the cause is modSecurity or similar measures, you may need your host’s help in resolving this.