This is probably do to a PHP error happening during the AJAX call.
Add this to you’re wp-config.php file if it’s not already there, or alter as needed.
define('WP_DEBUG', true);
define('WP_DEBUG_DISPLAY', false);
define('WP_DEBUG_LOG', true);
Now try saving a post and after a bit download the error log which should be located at /wp-content/debug.log and look to see if and what errors are happening.
