I'm having the same problem. Won't let me reister my device or read the book I bought on computer or on my Nook. I declined to register when I first opened the book. It let me open it that time, but never again. Needless to say, I'll never make this mistake again.
I found the first steps to solve the problem, might be that it helps you as well! This error message means that there is another ID linked to your computer.
To delete the "old" ID use Ctrl-Shift-D. To re-authorize use Ctrl-Shift-U ...