It's a cat and mouse game between youtube and the developers of the download engine, youtube-dl.
The built-in updater for the engine iften fails, especially in media downloader version 2.x. I download youtube-dl from its own download page and put in the engine folder manually, replacing the existing copy. It's a little work and not for the non-technical user, I suppose, but it works for me every time Media Downloader stops working.
Hth,
George