No, it hasn't, but thanks for the suggestion.
I did log out once on the 2.0 update, but I could log back in on the device itself. Now, even though I've connected with the Kobo Desktop and let it go through its (incredibly slow!!) machinations and "ejected" the device when all was done, the device on rebooting still says I'm not logged and wants me to reconnect to the Kobo Desktop. I seem to be stuck in a loop.
2.4 seems unreiiable to me. No-one wants to suggest what might be a stable firmware? Pleaser?
P.S. I don't think the book is faulty. It works fine on both Sigil and ADE.