So it is a 'pure' ADE or Adobe DRM infrastructure issue, and does not appear to be related to the Nook itself.
Did you try the suggestions I made near the top yet?
- Make sure the book is not genuinely expired (if a loan).
- Double-check the time and timezone settings on both the computer and the Nook.
- If neither of those works, try the older but more reliable ADE1.7.2.
If none of those helps I can't think of anything else right now, even with all the extra information.