Thank you so much everyone for your helpful replies and suggestions. I'm sorry I actually re-posted this question thinking that my post had not been published as I couldn't find it on the forum. But then I found responses in my email! I don't know how to delete the double-up post I posted yesterday?
Kovid Goyal thank you - your simple suggestion to update Calibre worked! I am no longer getting that error. I must have already had Calibre open and not updated it before updating the Kobo firmware and encountering the problem, even though I had updated Calibre recently. It hadn't thought of the possibility that a recent version of Calibre might not be compatible with the latest version of Kobo firmware. Thank you Kovid Goyal for developing Calibre
But if I have problems with the new 5.34 Kobo firmware in future I will revert to 4.30 instead, thanks for the suggestion Poppaea. It would seem that there would have been many versions between 4.31 and 5.34 - seems strange that Kobo hasn't resolved the bugs that started with 4.31 even with 5.34
Compared with my version of Kobo firmware from 5 years ago, I am finding this latest 5.34 version much better in terms of ease of highlighting. That's so much easier. But it's a bit annoying that it takes you to a home page with annoying sub-headings like "fave" "find your book in the Kobo store" etc. I liked my very old version that just took me to Books
Thanks again everyone!