What does 'The response is not a valid JSON response' error mean in WordPress?

This error typically indicates that an AJAX request made by WordPress (e.g., when saving a post, uploading media, or using a page builder) did not receive the expected JSON data back from the server. Instead, it received something else, such as an HTML error page, a PHP warning, or an empty response, which the JavaScript client cannot parse as valid JSON. This often points to a server-side issue or a conflict preventing the proper AJAX communication.