See this lengthy thread discussing several *new* issues with B&N:
https://www.mobileread.com/forums/sho...d.php?t=246519
tl;dr -- things changed, like, a lot.
FWIW, the latest updates to Apprentice Alf's tools should fix the issue, which mainly deals with finding the right encryption keys.
Further questions can be directed to Alf's blog, the comments section there is where DeDRM support is provided.