Skin + layout files gone after publishing page again

Somehow, every time I re-publish (update) a page, the theme layout is broken. We are making use of wordpress with multiple pages (network).

I’m missing the following files in particular in the folder uploads/sites/13/bb-plugin/cache

  • 4-layout.css
  • 4-layout.js
  • skin-xxxxxxxxxxxxxxxx.css (whereas xxxxxx seems a timestamp)

What I figured out so far, is that 4 stands for page ID 4.
My folder permissions for uploads/sites/13/bb-plugin/cache is 755.

the upload path in the database table wp_13_options is empty.

Any help is much appreciated!