It is probably down to a memory of the old registration somehwere, either on ADE (Digital Editions) or the Nook or both.
Try the following: Some of the steps are probably unnecessary, but depends on the details which ones.
- ctrl-shift-D to ADE to deauthorize ADE itself (cmd-shift-D on Mac)
- ctrl-shift-E to ADE with the Nook plugged in to deauthorize the Nook (cmd-shift-E on Mac)
- restart ADE
- authorize ADE with your new email and password (menu-Help-Authoirze Computer)
- copy the authorization to the Nook (menu-help-Authorize Device)
There are quite a few bugs in ADE2.0.
If the above doesn't work, it may be that the Nook update has somehow stirred one of them.
In that case, try the older more reliable 1.7.2.
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There are lots of bugs in ADE2.0 (and 2.0.1). Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Some people have found ADE trying to upgrade automatically.
It appears (not 100% sure) that if you install ADE2.0 as a new install (not as an upgrade) that your 1.7.2 will continue to run.
Probably best to say no if 2.0.x installation asks if you want to migrate your library.