I suddenly got this error message using Calibre version 3.48 on my new Libra 2 when I installed the latest firmware 4.33.19608
Noticing many reports on mobileread that this version was buggy, I downgraded to 4.31.19086 and it all works fine.
It's all working well and there's nothing extra in 4.33.19608 that I need so why upgrade?
I should add though that if I hadn't changed to Apple, I'd still be on Windows 7, and will not now upgrade my Kobo firmware ever, or until I'm forced to, so I may not be your typical customer