Getting the same error, would love a solution to this!
I’m seeing the error as well. The code we use to pull the featured image has been in production for over six months without an issue. I found that the issue is only happening on one post so I dug in a little deeper to see what was going on.
I can’t seem to find the issue causing “json_user_cannot_read” to be returned in the result set but I did get it to work.
Since I could only reproduce this on one post, I located the featured image in the media library, pasted the URL into a browser and rendered the image to make sure it was working properly. Then, I saved a copy of it locally, re-uploaded it to the media library and associated the *new* image as the featured images to the same post. After that, the JSON API returned the proper asset information.
I don’t see any difference between these two images but maybe there is some underlying meta that is causing the issue.
I wish I had a better answer for this but at least it may help diagnose the root of the problem.
Here’s the answer: https://github.com/WP-API/WP-API/issues/1526
It’s (IMHO) a bug that ONLY affects posts where the featured image ISNT attached to the post being fetched. So if you re-use uploads as featured images the second/third post to use it will have issues.
Re-uploading predictably fixes it because the new upload is attached to the right post.
