I have the same problem, now 3 incomplete movies on my Ipad. They did charge me for them of course. I contacted apple twice already with the "report a problem" link in Itunes, but that has been a waste of time until now. They suggest installing the latest Itunes version and try again.....on my Ipad?
Looks like a copy/paste answer to me. This is extremely frustrating. I have learned that when you download a movie and it stalls, you pause it, and then resume it, it will continue to download. But you have to keep your eyes on it all the time.
But if I let it time out, or are am not patient enough to wait 2 hours for the thing to download, then I get the download errors.
Very disappointing Apple, expected better from you guys. Maybe a reimbursement for the rental fee paid and in the next update a final solution? O yeah and a way to free up this space in the mean time.