This is just a guess, but it sounds like a PHP error on thee server during the AJAX call is causing it not to return anything, or it’s outputting the error.
Turn on debugging on your site
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);
After turning on debugging try to save the post then look at the log, it should be located at /wp-content/debug.log
See if there’s any errors and what they are.
