Images have vanished

Scouring Google for the past couple of weeks I see that my problem is far from unique in itself, but apparently is in what is needed to solve it — none of the solutions of the past have worked.

I have a multi-site, up to date site at https://universal-nexus.com
that site is working flawlessly. all of the subblogs … not so much. for comparison please see https://sandbox.universal-nexus.com — a snapshot of the main site from several months ago I made for testing new plugins and themes before putting them live on the real site.

Things I know aren’t the solution:

• Permalinks, as you can see are working. They had quit, but I got that solved.
• The permissions of all folders in question are 755 and all files are 644 and changing them to be more open broke things worse
• I have confirmed with my host that hotline protection is not something they have nor is it anything they would turn on arbitrarily.
• I have not only disabled all plugins from Admin, I have disabled all plugins from MySQL, and just to be safe I nuked my plugins directory.
• the main site is running Twenty-Eleven theme. The sandbox site is, iirc, running Twenty-Fourteen, it a few days ago was on Twenty-Ten, none of that mattered.
• Without doubt the images are there and intact. They can be browsed to and worked with without issue. I can also go to the media library and edit the image and it shows up, but any other part of the media library or the site they don’t.
• New uploads are instantly affected
• I have edited the media upload location, which is listed as
  

  wp-content/blogs.dir/8/files

  by a) adding a / in front b) clearing it and retyping it

• I have only the root .htaccess file which currently reads:
  

  # BEGIN WordPress
  <IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule . /index.php [L]
  </IfModule>
  # END WordPress

• I have tried toggling the option to organise media by year/month — nothing changed, even on new uploads

I think that pretty fairly covers all the logical things I’ve tried. I’ve probably tried more than a few illogical ones, but by now I’ve forgotten what they were, suffice to say they didn’t work so I set things back.

I have manually reinstalled WordPress.

I’m not really certain what else there is to try. I have backups, and I could restore from them and only lose a few changes (this would have happened between two backup schedules, of course) but that really seems overkill for what I’m certain must be a very simple fix if I only knew what I was looking for.

FWIW I did have W3-Total Cache installed at the time everything went mad, and it was being stubborn about disabling and going away. I can’t see how it would be impacting anything, but one never knows.