YouTube's ContentID system blocks videos alleged to infringe on copyright. Lately it has been coming down on a great many highly visible, legitimate videos. YouTube is apparently taking its time in resolving the problem.
Several popular YouTube accounts, including those belonging to 'MIT OpenCourseWare' and the 'Blender Foundation,' have had all their videos blocked. People who try to access the videos are informed that they are not available in their country, suggesting that YouTube's piracy filters have been triggered. It's unclear, however, who or what is to blame.
Source TorrentFreak : YouTube’s Piracy Filter Blocks MIT Courses, Blender Videos, and More (Updated) [torrentfreak.com]