Why are the preview thumbnails blank when I try to use a LinkedIn link?

I have an article on the website that I help maintain that I want to share with LinkedIn through the Share Update form on the site. Possible thumbnails are detected and LinkedIn gets the correct URLs for the corresponding images (served by Amazon S3).

Inspecting the page, I can see that the call is being made at https://www.linkedin.com/sharing/api/url-preview and the JSON response includes a "previewImages" field (under "data". "Content") that represents is an array of objects / dicts with fields "url", "mediaProxyUrl", "width", "height" and "size".

The "URL" of my preview images is correct. Copying and pasting into the address bar brings it up. However, "mediaProxyUrl" does not load the image. The size field is null.

Using a working link url (article on another site) I can see that "mediaProxyUrl" should be linked in the url for the thumbnail, and "size" should be the file size of the original image.

So why are my preview thumbnails blank? Is this a problem with the end of LinkedIn? Amazon s3 problem? I'm at a loss.

PS I checked the headers og: image and og: image: secure_url, they are ok.