Elementor shows a red toast when you try to save a page. The message reads “Failed to save: The response is not a valid JSON response.” The editor stops the operation and reverts to the previous version.
The editor uses the WordPress REST API to send the page data. The API must return a JSON payload. If the server returns HTML, a redirect page, or an error page, the JavaScript parser throws a syntax error. The result is the JSON response error.
Typical triggers include a firewall or security plugin that blocks the wp‑json endpoint, a mismatch between the site URL and the protocol used in the admin, an unsaved permalink structure, a PHP memory exhaustion error, or extra output from a theme or custom snippet. Any of these changes the response from JSON to HTML.