iTunes won't download videos - "must be connected to WiFi" ???

Having a very strange iTunes problem that I've just noticed for the first time. Whenever I try to download a purchased video from the cloud, whether it be a previously purchased movie or TV show, I get the following message: "This Item Cannot be Downloaded on This Network -- you must be connected to a WiFi network or download it on your computer."

I would expect to see this while downloading from an iPhone over 3G, but this is on my home iMac connected to my cable broadband internet via wireless-N connection. Very very strange. I have not yet tried to download a new video purchase -- only previously purchased videos from the cloud that I no longer have on my hard drive. iTunes still downloads large app updates (over 50 MB) just fine. Anyone ever encountered anything like this?