“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.1 You can also go to the Tag Groups settings -> Back End -> Gutenberg and disable the block preview.

Footnotes

  1. 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.[]