Thanks for the pointer but no permanent solution there. I can't be the only one having this can I?
I emailed Kobo yesterday at 4 pm and received a reply at 5:41 am(!) today. I'll try this the next time it happens (and I expect it will :0 )
Here is what Erin said:
Thank you for contacting us.
To fix this have please plug the Kobo into the computer and open the Desktop. Then click on the e-mail address and sign out. Wait 30 seconds and then sign back in, update then sync to correct the error.