I don't know if the issue I have with my book is to do with this new firmware version or the book .....

(What I do know is that instead of continuing to sit down and read the book I'm now on the pc and telling you lot about my problem, lol!!)
The book: Paul Fussell's The Great War and modern Memory. I bought it from Kobo as an epub2, so I didn't need to convert it when I put it into Calibre.
I've gotten about 90 pages in and whammo I can no longer read the book.
What happens is that every time I go to turn a page the book closes to the device home screen showing that it's finished.
So, at various times I've gone into the book and opened the Table of Contents and successfully moved to the next chapter (from where it got stuck), and when I swipe to move to the next page the book closes and goes to the device home screen. I've also tried opening the book using the page slide bar, and irrespective of what number page it's supposed to open at, it actually opens at page 53 (or thereabouts), and then bombs out when I swipe to go to the next screen.
I've also done a pin re-set and nothing changes. In fact the book comes back up showing finished.
I've also deleted the book from the device and re-loaded it afresh from Calibre - again I can't get past either page 1 or whatever chapter I go to because when I swipe the book closes and goes back to the device home screen showing book finished.
I've checked the pages of the book in Calibre and can successfully read the area where the book fell over, and anywhere else for that matter.
One thing to note is that earlier in the book from where it fell over I had made two highlights. Before I deleted the book from the device I used the Calibre feature of moving the Annotations into Calibre - and that happened successfully.
So what the darn is going on?????