I've also posted a report on Launchpad for KoboExtended under the exiting bug #1258521 "Converted epub causes Kobo Glo to freeze, when reloading the converted book"
https://bugs.launchpad.net/calibre-k...r/+bug/1258521
I'm really starting to think it's how the epub is put together. I mean it passes all the checks I've done (Flightcrew, epubcheck, some online one I found) but perhaps it's the Kobo that isn't supporting certain things.
Is there a validator that's available specifically for Kobo devices?