Thanks for writing in! X/Pro themes utilizes default WordPress menu structure. If you have a test site, you can disable X/Pro themes by activating another theme and re-test this issue to see if it is related to X/Pro themes.
Most probably your issue is related to PHP memory limit or it could be related a plugin conflict. To increase your PHP memory limit, please refer to the following detailed instructions here (https://theme.co/apex/forum/t/troubleshooting-increasing-php-memory-limit/16935).
To test for a plugin conflict, you will need to disable all of your 3rd party plugins and re-test your issue further. If the issue resolved, you can then activate your plugins one by one to see which plugin causing the issue.
Also make sure to clear all cache when testing your issue. You can refer to the following resource here (https://www.wpbeginner.com/beginners-guide/how-to-clear-your-cache-in-wordpress/).
Let us know how it goes.