No-one seems to be answering these questions but I managed a partial solution myself - My computer had been registered without an Adobe Id. It was necessary to de-activate that Id by typing Ctrl-Shift-D. Then I could activate with my old ID (email address) and password.
This enabled me to then download another book and transfer it to my Nook. The ebooks that I had earlier downloaded but not able to copy were still unavailable as they had been downloaded to my earlier, incomplete activation. I deleted these and now have to wait for them to expire in 20 days before I can download them again to my new activation Adobe Id.
However, I can now download new books.
Frustrating and annoying that there is no support for this product and no other alternative. Perhaps someone else knows of one?
Having the same issue and glad to have at least a partial answer. Hate that I will have to wait for the borrow to expire to download again as I wanted to read the book. Oh well, at least I'll be able to read something else now! Thanks for the info!