Recently I ran into this error when exporting my music video from iMovie to Youtube. After looking up a few solutions online, I had determined that this was caused by Youtube, not iMovie. To get around this issue, don't share from iMovie. When you go to share your movie with iMovie, instead of sharing with youtube, share/save this as a file to your documents. Once you get the message that your file as successfully been made, go to youtube and upload your video there. I've tried it and this works!
