I replied directly to your posted question this morning.
This error is indicating that there was a 500 error response from the admin-ajax.php on the login page.
You can disable the login security by removing the first line of code that was added to your wp-config.php file.
You should also check the error_log files on your server to see what this 500 error is about.
If you want me to help you fix this directly you can email me with your FTP login and I’ll do it for you.