Sorry I can't answer your question but I had the same thing happen yesterday. I was testing FBReader after installing this version -
FBReader_180d. I have a couple of books that, when I try to open them I get an hourglass in the middle of the screen for several seconds then I am back at the list of books view.
Since none of these books were originally DRMed, I don't know why they won't open with FBReader...but it doesn't bother me. Maybe someone else has an idea what might cause this but you aren't alone with this oddity.