Apparently it is the moment that I edit the metadata in Calibre that the EPUB file is broken for some reason. I changed the name of a book from "A Game of Thrones" to "Song of Ice and Fire #1: A Game of Thrones" and the book stopped loading, giving me that error dialog box. Anoyone have any ideas? Like I said before, the original "A Game of Thrones" book loaded fine, but the minute I changed the title, error on my Nook HD+. This is frustrating, you'd think they'd have this crap figured out already with how long this device has been on the market.
You said "I did nothing to the original EPUB, just merely added a cover to it in Calibre "
Well, then, you did something to the original epub. Calibre has to modify the original epub in order to add this cover. The trouble is, you have no idea what else Calibre may've decided to "fix" in the process.
Calibre is an awesome tool and I use it when I must but it's a little too heavy handed for my tastes. It does a little too much even when nothing needs to be done and I rarely care for the results. I repair/modify/improve my epubs manually using Sigil.