Looking though the music threads on here, some of the videos display properly and some don't, they have what is like a grey stop sign over the play button. I can't see any obvious difference between the videos that work and those that don't. As far as I can tell they are all you tube vids.
Something to do with how the video is encoded or something, after googling about it seems that you can watch these videos in the youtube app on your ipad but not always as a link, especially for the older vids. I'm going to try typing one of the videos I can't see into the youtube app and see what happens, I'll get one off Abba thread.