I am getting the same error on all of my epub books; some of which I was able to successfully load and read on my nook just a few days ago. I am able to read them from the device in Calibre but as soon as I disconnect the device, they do not show up in my documents and after plugging the device back in and attempting to read from the device in Calibre again, this error appears for all of my files. I have run all of my books through the conversion (epub to epub) just in case there was some corruption but the error remains. Even copying PDF files to the Nook results in failure although I can read the first few pages, the remainder of the content is blank. I can't believe that all 180+ of my books have spontaneously corrupted. Have you any suggestions?