Either your computer or the nook has probably forgotten (or confused and half-forgotten) the authorization.
Try deauthorizing the Nook: ctrl-shift-E to ADE (cmd-shift-E on Mac) while the Nook is plugged. Then restart ADE and reauthorize the Nook.
If that doesn't work, try the same with ADE itself. Deauthorize (ctrl-shift-D to ADE (cmd-shift-D on Mac)), restart ADE and reauthorize ADE.
Also, check that the time and time zone still match up between PC and Nook (if it is a Nook that knows the time).