Am having the exact same problem: cannot save to anything, e.g. file, Vimeo,
Called support and they are equallly baffled. Scanned this site for soluton and found we're not alone. Not sure if Apple even aware. Support had me download files for analysis by engineers. Am hoping for solution in 3-5 days.
In meantime, made the exact same video with iMovie 9 using the exact same clips from the same source and had zero problems.
BTW if you haven't seen iMovie 10, its appearance is drastically different from its predecessors. It's considerably more difficult to navigate, call up transitions and adjust clips. Audio fades, while possible, are very difficult. Could go will stop in the interest of brevity.
In short, will post solution here if/when Apple comes up with one.