Thank you for reaching out to us. Can you 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.
If you're using a caching plugin, clear all caches including browser cache then deactivate your caching plugin 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' );
If you still have problems kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case.