While I was under the impression that this only happens if the downloader gets too many items at once, I encountered it today while trying to verify the installers for The Witcher 2. What solved it was adding something else, so try adding one of the free games, preferably the bonus content, and see if that helps it.
Oh, and do check that the path the downloader wants to download to exists as well, you will get trouble when adding an item to the queue and the download path doesn't exist (removed disk for example).
Thanks for the suggestions, but none of those things are my problem. I tried pointing to a different folder, just in case, but it made no difference. It happens instantly, as soon as I click a download link in my collection page. I've tried installers and bonuses, but no change.