It sounds as if ADE has got confused registration.
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0
Another possibility is that the registration on the Nook is corrupted. In that case, plug it into the computer, run ADE, and use ctrl-shift-E (cmd-shift-E on Mac) to clear the Nook registration.
Restart ADE, and reauthorize the Nook (menu ~ Help ~ Authorize Device on ADE2.0, menu ~ Library ~ Authorize Device on the superior ADEv1.7.2)