theres been a recent Youtube redesign, but I've not heard of it breaking anything like this
Must admit I've never liked that "Youtube Downloader" - somer versions of it (at least) come with embedded spyware and I've had to clean up a few resulting messes in the past
Best Youtube download solution as far as I can see is still simply to install the free version of Realplayer and use the download button it places within the Youtube video frame