I have the extension installed on dcwwiki.org and Download QR code button in the left sidebar works nicely. However, when I try to find Special:QrCode, it results in "You have requested an invalid special page." I have enabled $wgUrlShortenerEnableQrCode as true. Any suggestions what is potentially leading to this?
Topic on Extension talk:UrlShortener
The UrlShortener version you're using (MW 1.41 release) does not have the special page, because it was not added at the time. When it was added, MediaWiki was already at 1.42. You should upgrade the extension.