I am having the exact same problem. I just downloaded the Nook app today (12-27-2011) and bought my first book. Any suggestions would be appreciated.
I was having the same problem. Here's how to fix it:
Uninstall the Nook app and reinstall it. Worked for me.
I have tried signing out and back in again. Didn't work. Then I tried deleting the app and reinstalling it. Still didn't work. Any other ideas?