What am I doing that's tripping this false positive? It's not clear what I need to change.

Also, just curious - does Preview fail for everyone else too? You can't get back to the editor. You've got to start over with uploads etc. I'm Linux-Firefox, with Privacy Badger & uBlock Origin & HTTPS Everywhere, so it's probably my fault.

EDIT "Seems to be a recent issue. Some users reported success by editing the post and adding the images afterwards."
Re: Bought these (retro) hardware today