Hello Patrick,
We have moved the Theme’s customizer from Appearance > Customer to X > Theme Options which is dedicated to the theme settings away from the WordPress customizer options and additional settings.
The loading error of the Theme Options is not related to the changes of theme. It is because of the following:
1.) Your PHP needs an update.

Please make sure that you have meet the requirements to run the latest WordPress core. You need at least PHP 7.2 to run WP 5.x.x. For more details, please check this out:
2.) Your outdated WordPress core is not compatible with the currently installed theme version.
Our latest versions are:
- X theme 7.1.2
- Cornerstone 4.1.2
- WordPress 5.3.2
3.) Your PHP Memory limit needs to be increased. We recommend that you increase your memory allocated to PHP. To increase it, 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. */
We would loved to know if this has work for you. Thank you.