I'm having the same issue. Saw some previous posts that I thought might help but I've gotten nothing. Very frustrating. Hoping for a fix soon!
I'm having the same issue, but with my Nook app on a Samsung Galaxy Note 3. I can read the books (or at least, one of the two I've bought with ADE) on the Aldiko reader on the same phone, but not on the Nook app that I just downloaded -- even though the books show up there! I get a message saying "Could not open. User is not activated on ADE."