This has happened to me after an update I got yesterday. Only the book I was currently reading unfortunately. Tried all the remedies suggested on this thread but no go. Deleted the book and downloaded it again and ran it through ADE but still no go. Finally gave up and deleted book again. This is the only problem I've had with the reader and it is disappointing to say the least.
It didn't matter whether I loaded to book via Calibre or ADE. Using Mac software and also updated Kobo Desktop as well.