Wordpress - Elementor Plugin - The response is not a valid JSON response OR The preview could not be loaded Not Found 404 error

When you try to  edit a page using Elementor plugin, you might come across the following issues

 

To get rid of this,

1. Change Memory setting and increase memory to minimum 256M

In wp-config.php file add the line

define('WP_MEMORY_LIMIT', '256M'); 

2. Change permalink setting to Plain.

That should do the magic.