You may not have deauthorized ADE 1.7.2.
If you haven't, try launching ADE 1.x and hold ctrl+shift+D keys to deauthorize.
Once you have done it, try authorizing ADE2.0.
If you're still facing issues, contact technical chat support, for step by step assistance.
This issue can be resolved by deleting adept key from Windows registry.
I have a similar problem, ADE sees the Nook (in my case, a Color Nook) but will not authorize it. I am getting the same error message. However, in my case my old authorization was with a now-broken Mac and my new authorization is with a newish Mac 10.7.5, which does not have an old ADE on it. Anybody have any suggestions?
Thanks for the help. Not sure if I ever had ADE 1.7.2.... What worked for me (for what it is worth!) was to deauthorize my computer. I opened ADE, and then held ctrl+shift+D (as recommended). I then restarted my computer, hooked up my new Nook, opened ADE and reauthorized my computer. ADE then allowed me to authorize my Nook. I am now reading my e-books that I purchased for my Sony E-reader from the Sony store on my new Nook. Many thanks to Yuvraj for the help! Hope this helps TouristinTheCity.