Thanks for writing in! First of all, please check your PHP version running on your server and make sure to use PHP v5.6.x or later to avoid incompatibility issues. Also to check 500 error details, you can enable WP_DEBUG mode on your WordPress and re-perform the same actions (https://codex.wordpress.org/WP_DEBUG).
Also make sure that you're fully updated as well. You can check the latest version numbers from here (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195) and update your X theme accordingly (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62).
If you're still having issues, you need to test for a plugin conflict by disabling your third party plugins. If the issue resolved, then you can activate your plugins one by one to see which plugin causing the issue.
If you cannot isolate your issue, please provide us with your WordPress login credentials along with your FTP credentials in a secure note to check your issue further.
Let us know how it goes.