For some reason, NOOK 1.5 will no longer open files that have two dots in the filename. If you click the "Download" button on an e-book in your B&N account, B&N will name it something like xxxxx_epub.v2.epub and that dot ahead of the v2 gets 1.5 all annoyed (it worked fine on 1.4). Apparently some wireless downloads also got double-dot names earlier last year.
For sideloaded files, take the file off your NOOK, rename it, and put it back on, and it'll be fine (renaming it
on your NOOK might not work. Or you could just use Calibre, which insists on renaming everything.
For wirelessly downloaded files, archive it, unarchive it, and redownload it.