Last modified: 2007-05-23 08:01:57 UTC
Images which have a plus character in its filename don't show as thumbnails, only the full sized image shows. Probably it's a problem with the URL encoding of that character and the subdirectory used for the thumbnails. Additionally, the check usage function does not work because there the plus gets converted to a underscore, which is of course a different image. The problematic image is commons:Image:Ferdl+franz lang.jpg
Note that CheckUsage has nothing to do with us; report problems with the tool to the tool's maintainer, Duesentrieb. That doesn't mean this isn't also a bug in MediaWiki, though.
Seems to be fixed in the meantime, somehow. Thumb is created and visible on the page.