Thanks for the details!
I am able to login to your site and can confirm the issue.
Can you please confirm if you have checked the troubleshooting process given by @christian_y above in the post.
I am adding those here again. Also please check your PHP version of your server.
Ensure everything is up to date. You can see the latest version of Pro in our Changelog at https://theme.co/changelog/. 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.
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.
Please switch to the parent theme to see if customization made in the child theme is causing an issue.
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.
5.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 could please try the above and if you still have the issue we'd be more than happy to assist.