Gutenberg editor when you attempt to create or edit a page or post or add an image and finally "update" you may see one of these errors that blocks the update:
The response is not a valid JSON response. (Most common).
Not found. (Server side)
404 (Not Found) (Server side)
The fix is simple.
In WordPress Admin Menu go to Settings > Permalinks and click the button "Save changes" without making any changes. Done!
This can also happen if you previously added a free SSL certificate before WordPress 3.5.1.