Nook really doesn't like "#" in the file name, eh?
I transferred some books to my Nook Glowlight 4 Plus, and they all had #'s in the titles as part of a book series. My Nook sees the books in the library, but I get errors when trying to open them. I removed the #'s in Calibre, which changed the .epub file names, and sent them to my device again. Voila, they work perfectly. It's a little weird that they dislike the # symbol so much. They probably only show up in the library due to the separate cover.jpg file that Calibre puts in the book folder.