Thank you Mike!!!
Yes, that was the problem. Apparently she drafted the post in a private conversation, then copy-pasta'd the contents into a public post. So the embedded image did not get the correct permissions because it wasn't re-uploaded.
Would be good to have a more informative error...
I just confirmed that the author of the post CAN see the embedded attachment. She logged in using my same browser, in a Chrome "incognito" session.
Also confirmed one other forum member CAN see the attachment.
When logged in to my account I can NOT see the attachment.
In the apache log I see...
Another user (admin) made a post with an embedded image (attachment).
When I (also an admin) load it I can view the text, but I get a broken image. Inspecting the element shows a 403 error.
She says it shows up fine for her. How can I debug further?
Possibly relevant: she also has a problem...