From my original post:
"Epubcheck gives it no warnings or errors."
It would be helpful if there were a list of why adobe digital editions might give this error message. Does such a list exist somewhere?
I had this same problem. epubcheck that I have on my desktop said no errors, but ADE would give the "minor errors" message. I put some serious dents in my forehead from pounding it on the desk.
I uploaded my ePUB to http://www.threepress.org/document/epub-validate/ and let it validate against schemas that I don't have and it gave me the clue that I needed. Because I had exported my book from MS-Word using their awful HTML export and then (through a series of ordeals) converted it to XHTML and cleaned up all the uglies. But, there was one piece I forgot to fix. The encoding value in the top-level xml element. It was set to Windows CP1252. It has to be UTF-8 or UTF-16. When I changed the encoding, I had to fix some of the quote marks that I had missed in earlier passes.
After making the encoding change and the final cleanup, I now have my book on my nook.
I hope this helps someone else in the future.