Please check that you have all the plugins updated to the latest version. If you have everything updated, try testing 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.
If he issue persists, try to to have the PHP memory limit of your site increased.
To do this, please login through FTP then edit your wp-config.php file and insert these lines:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
You can add it before the line
/*That's all, stop editing! Happy Blogging. */
Here is a details article about the PHP Memory Limit that is in our Knowledge Base:
In case the issue persists, please try to get in touch with your hosting provider and confirm to them that your PHP memory limit is increased.
If you are still having an issue, please provide us with the admin details of your site in a Secure Note so that we can check this further.