“Error loading block: The response is not a valid JSON response.”

If you see this message inside a Gutenberg block then your web server sent a response that cannot be understood by the editor.

Advanced users can check the developer tools in their browser. You may find there a message that gives you a hint. A common cause is a firewall on your server that gives a false positive:

If you cannot change the firewall settings, you could try the preview button if your post or page still renders properly.((Since the front end doesn’t transfer the data the same way as the editor, it most likely won’t be blocked by a firewall.)) You can also go to the Tag Groups settings -> Back End -> Gutenberg and disable the block preview.