When editing pages or templates in the backend, particularly if they contain Views, you may experience problems with the interface not working as expected, and this may be accompanied by reports of JSON issues in the browser console, and possibly warning messages on screen that “The response is not a valid JSON response”.
This occurs when Yoast is active, and may affect other SEO plugins, when an image block using a dynamic source is included in the content.
Then you may see failed GET requests in the console such as
On some hosts the presence of single quotes in the request can be interpreted as malicious, which may trigger blocking of other requests that then affects the funcionality of the editor.
The developers are aware of this issue and will address it soon.