YouTube made some changes to their system on 09/13/12 that broke most of the download apps. I use Download Helper and here is what they say.
Quote:
We are aware of the problem that occurred on Sept 14th (13th if you are in the US).
This is due to some changes YouTube just made. We are working hard at solving the issue. Please, give us a little bit of time and refrain from downrating the addon.
UPDATE: we now have a temporary workaround. It is available from
http://www.downloadhelper.net/install-b ... n=4.9.10b6