This is not an uncommon problem. It happens quite often and the problem is not limited to Movie Studio.
The problem is that YouTube keeps changing its upload system. And when it does it messes up all the editing programs that are designed to upload directly to it.
You could wait until Magix figures out what's up and issues an update. Or you could take FayFen's advise. Just output your video as a 1920x1080 MP4 and upload it to YouTube through your browser. It may seem a little inconvenient, but it takes no longer than having the program do it for you.