Edit: This was symptomatic of a much broader issue. For more information, refer to the post WordPress sites not available externally.
It seems overnight that images from the media library are not loading on any of the WordPress websites I manage e.g. basil.udance.com.au, sachika.udance.com.au, blog.udance.com.au, readymcgetty.com.au, etc.
Here’s an example of an affected post.
The issue is independent of themes as I run several themes. I also tried switching off all third-party plugins to no avail. After much frustration, I narrowed down the issue to JetPack.
Here’s the relevant section:
If I disable the switch Speed up image load times, images load properly.