I've just noticed the same thing. Funny thing is: yesterday everything went right for me, and now I suddenly have this problem with one particular book.
I see they've put a notice on their help pages about it. Which doesn't suggest they've found a fix yet...
https://help.kobo.com/hc/en-us/artic...ital-Editions-
Maybe it only happens with certain books?
Or is it the case that once it has happened to you with one book, from that moment on you're no longer able to download any book from Kobo anymore?
Note that another ebookstore had this to say about the same problem 6 months ago:
https://support.ebooks.com/hc/en-gb/...DEPT-NOT-READY