It's technically illegal to save Youtube videos to your hard drive. So, in order to keep on the right side of copyright holders, Youtube keeps changing the way their site works in order to thwart the use of software such as Ant Video Downloader, which does seem to have stopped working (until an updated version is brought out) with Youtube for the moment.
So if you want to keep saving Youtube videos you have to be prepared to join in the cat-&-mouse game of finding out which software works with the latest version of Youtube. (As soon as you find something that works there's every chance that Youtube will revamp their site to try to prevent it from doing so. That's also why you'll struggle to find any Youtube downloader extensions for Chrome; Google own Youtube and they can't be seen to be supporting illegal downloads by providing the means to do obtain them via their own browser).
This currently works (extremely well) with Firefox:
https://addons.mozilla.org/en-US/firefox/addon/easy-youtube-video-download/