Help with Media Downloader

I’m been trying to download a few YouTube videos after seeing Media Downloader. It’s only worked 2 times so far. I can only successfully download videos when I am given file sizes on the list of video formats to download when clicking the extension button. Other times, instead of a file size, it’s a blank line. Clicking that will cause the download to fail, saying “Server error”. I’ve tried disabling extensions, but it does nothing, just as I expected, since the 2 times the download worked, the extensions were on.
Is there any way to get the download to work? I haven’t checked if this question was answered already or not.