Images that are uploaded locally to the private wiki wikipedia-pl-sysop.wikimedia.org fail to generate a thumbnail.

If image is embedded into wikitext with the following syntax:

[[File:Filename.png|thumb]]

It results in a broken image (presented below). The src is (AFAIK) correctly set to /w/thumb.php?f=Filename.png&width=220, but that URL serves an error page (as displayed below). Loading image via the img_auth.php endpoint works fine.