It's technically illegal to save Youtube downloads and Google has a legal obligation to copyright owners (such as the BBC) to try to ensure that Youtube videos can't be saved to your hard drive. So they periodically tweak the Youtube website to try to thwart the software which seeks to grab the videos they're hosting. That's why nearly all Youtube downloading software periodically packs up until, of course, the developers of that software modify it catch up with the changes.
So you might need to wait until YTD is updated or, of course, try a different downloading tool. The problem there is that most of the best ones are nowadays designed as browser extensions, rather than as standalone programs, and not many developers design extensions for SeaMonkey. (With Firefox, both Ant Video Downloader - which downloads from any video site, not just Youtube - and Easy Youtube Video Downloader Express 7.38 should work fine).