Recently I found for the congress wiki (also for last year's wiki), that thumbnails are not generated (files are present, but only have 3 bytes).
What can be wrong?
MW version is 1.31.1, and yes, I already studied some of the documentation. Strange issue. Similar in last year's wiki ( https://events.ccc.de/congress/2018/wiki/index.php/File:Wikilogo.png ).