This is a Marvin bug, because I get the same error messages about "damaged or encrypted" files when trying to load books from Google Drive. It used to work just fine in Marvin, but got broken at some point in 2016. But if you email the exact same book to yourself from that Dropbox or Google Drive location, Marvin will open it just fine, no problem. So, the bug is in Marvin's interaction with Dropbox, Google Drive, etc. – it got broken at some point, in that now Marvin calls perfectly legitimate book files "damaged or encrypted" if you're trying to load them from those cloud locations...