I assume the Nook is still correctly registered to your AdobeID.
A possible workaround (not always so convenient, but ...) is to copy the .epub files from the Mac to the Nook using the Mac Finder (or other file manager).
You should always be able to do the copy, and as long as the registered AdobeIDs match you should be able to read the books find on the Nook.
(If it fails, that it shows there is something odd in the Mac/Nook connection.)
- Borrow/buy books to get the .acsm file (as usual)
- Use ADE to process the .acsm and download the DRM protected .epub (as usual)
- Use Finder to copy the .epub (instead of copying using ADE)
- Read on the Nook. (as usual)