For two years I have successfully downloaded library books to my computer (Windows XP) and then to my Nook Simple Touch, using ADE. Suddenly, I am still able to download to my computer, and read on my computer, but when I try to move the book to my Nook I get the message "User Not Activated". What is going on and how can I once again read library books on my Nook?
Try deactivating the Nookm (ctrl-shift-E wjile plugged into ADE, cmd-shift-E on Mac) and reactivating.
If that fails, try a factory reset of the Nook.