Failure to receive an expected response from the server causes the ‘Not a valid JSON response’ error in WordPress.
Basically, WordPress needs to communicate with the server while you are editing a blog post. It relies on getting responses from the web hosting server in the background. This response is usually in JSON format, which is used to quickly transport data using JavaScript.
If, for some reason, WordPress fails to get the response, or the response is not in JSON format, then you will see the ‘Not a valid JSON response’ error.