It looks like YouTube Downloader (YTD) is having a problem in downloading YouTube videos recently. It failed to start any download with the following error presented: "Site responded with an error. Please try again later.". Fortunately, YouTube Downloader HD, another YouTube video downloader, remains working fine. So if you need to download YouTube video, you may consider using YouTube Downloader HD until a solution is available.
Official site of YouTube Downloader HD: http://www.youtubedownloaderhd.com/
Download page: http://www.youtubedownloaderhd.com/download.html