>>/920/
Hmm, I am not sure. Perhaps the API the downloader uses puts private URLs in a different location, even if they are now technically accessible. I am afraid I do not know much about the current state of pixiv parsing.
If this is a one-shot job, you might have luck messing around with either extracting your private faves' URLs as a list using your browsers developer tools, and then pasting them all in to a hydrus URL downloader page, or if you are ok trying out some simple hydrus parsing, perhaps you can figure out a way to autoparse that with a 'simple downloader' page and its very simple 'parsing formulae' to do the same job of just ripping a list of URLs the client is otherwise ok to download. Then you would just be pasting a bunch of 'private fave' gallery pages into the simple downloader page.