Did you try to upload your video to YouTube through Windows Live Movie Maker only to see it fail? It wasn’t just you! Turns out it was a change made on YouTube.com that caused this (authentication) failure.
This issue was brought to the attention of the Windows Live Movie Maker Team on the Windows Live Answers site. The team alerted the YouTube team who promptly rolled out an update to the service, restoring the ability to publish from Movie Maker.
So if your upload failed, try again now, all should be fine again.