Authorisation seems to work, but book won't open (black cover) on Kobo Aura HD

Trying to borrow books from the library. I get right to the end and everything looks like it's working, but book cover is black and it won't open. I get a message saying it's not authorised. It definitely is. I've experimented with erasing authorisation on computer and Kobo, re-downloading book, tying a different book, no joy. Sometimes I get lucky and I manage to download a book, but then I try again and nope. Anyone encountered this? I'm really reluctant to buy a newer one as there is otherwise nothing wrong with this. But I have wasted so much time trying to get this to work (are newer versions easier??)  I have done all the updates. On Windows (although interesting was previously on a Mac and had no issues....) Thanks