Thanks, what did the trick was adding $wgThumbnailScriptPath = "{$wgScriptPath}/thumb.php"; to LocalSettings.php
As you can see here: https://joinfediverse.wiki/Special:ListFiles
Since October many media uploads don't work as they should. Previews in different sizes, like thumbnails are not generated, so using the file in the wiki almost always leads to a blank preview.
So thumb.php works which suggests not a problem with image rendering.
Do you have 404 thumbnailing enabled? It looks kind of like what would happen if 404 thumbnailing was half enabled.
