Is it just one book that it is failing on, or does it fail with all books freshly downloaded from the library?
If just one book, maybe the Kobo has an issue with that book.
I found when I had a Bookeen Opus that there was a very small number of books it just would't read,
though the detailed symptoms were different.
If all new books, is there any possibility that you have somehow got the Kobo and ADE registered with different Adobe IDs?
Try copying a DRM book that works on the Kobo from the Kobo to the computer, and seeing if you can read it on ADE.
If so, you might need to deauthorize the Kobo (ctrl-shift-E while plugged in) and reauthorize it with the ID you currently use on ADE.
But be aware that this may mean you can no longer read obn the Kobo those older books that currently work there.