This is what I saw when I tried to open my next e-book selection (for the record: Kei's Gift by Ann Somerville). So, of course, this book suddenly became the only book I wanted to read out of all the hundreds on my Nook. I tried another book, and it opened just fine, so, if it was the Nook, it was only happening selectively. I tried another book by Somerville, and I got the same error message as before.
I did a bit of searching online and found a solution, although it took a few tries to get it to "take." Just in case it'll help someone else, here's what I did:
The problem was likely the file name. Almost all of the e-books I have by this particular author include a "#" in the file name. Just to be on the safe side, I renamed all files by this author.
However, when I put my micro SD card in, the book still wouldn't work. And, for some reason, it was showing up twice - one version gave me a screen that showed me the file path and that the file had no size (0 KB or something like that), and the other version gave me the same error as before.
One of the things I did in an effort to fix that issue was rename the file folders in addition to the files. The thing I think really fixed the issue, though, was this: turning the Nook completely off, turning it on, and then reinserting the micro SD card. Now the book opens with no problem and there are no "duplicate" listings of it.