[Ripplinger]

I only loaded 21 books when it happened to me, so that's not a reliable solution.

Edit: Actually now that I think about it more, it's possible I fixed it by removing all books and adding them back on again. But since this reader is just a spare to test out the firmware changes (and reaffirm why I won't put it on the one I actually use for reading), is the reason I loaded only 21 books.