If you have converted X to Pro and a child theme is active on your site, you will have to update the style.css.
If you are still getting an issue, try getting the PHP memory limit of your site increased.
To do this, please edit your wp-config.php file and insert these lines:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
You can add it before the line
/*That's all, stop editing! Happy Blogging. */
In case the issue persists, please try to get in touch with your hosting provider and confirm to them that your PHP memory limit is increased.
If you are still having an issue, please provide us with the admin details of your site in a Secure Note so that we can check this further.