I just upgraded to the latest version of WP.
I have tried using a few different versions of Flexible Upload.
I uploaded a pic to a post, published it, then link to it successfully from a page (published).
I uploaded a pic from that PAGE, and tried to link to the attachment/photo's own page from a thumb AND text link, several upload attempts with different versions of Flexible Upload... and I get a 404 instead of that IMG. It tries to link to /photo-desc-name/
When the one that works links to /POST-slug/photo-desc-name
OHHH!!!! I need a slug. And yes, it works with a slug. No need to upload new or anything.
I'm posting to help others out, I didn't find a solution.