Thanks for writing in.
There are many possible causes of this issue, it could be due to script conflict with other plugins or internal server error. We'll have to find out first which one is causing this, then we may able to provide some recommendation. Would you mind providing your site's URL and login credentials on a secure note?
For the meantime, please check for the following first:
Ensure everything is up to date according to our version compatibility list at https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195. Please follow the best practices when updating your theme and plugins. See https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62 for more details.
Clear all caches including browser cache then deactivate your caching plugins and other optimization plugins.
If you're using a CDN, please clear the CDN's cache and disable optimization services.
Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Reset your htaccess file by renaming it to
.htaccess-bak. Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
Contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Then, if you're on latest versions, then you may check this as well https://theme.co/apex/forum/t/troubleshooting-builder-preview-warnings/14404