A million thankyou's to Mumbletop
I had the same problem after (finally) managing to upgrade to 2.4. After trying all the more complicated procedures given on this thread, I tried your much simpler one and.... it worked!
I was devastated that I couldn't read my library books, nor a couple of books that I've bought from other websites, as this was the main reason I bought my Kobo Glo in the first place, so now I'm happy again.
Thanks again!