Well, the scrambled epub kindly provided by JSWolf had the gaps too, although to a lesser extent than my other books. But I'm happy to report that pasting the widows/orphans setting to the conf. file seems to have done the trick.
No more gaps! Still, it makes one wonder why my Kobo ignored the exact same settings in the css after the update...
Anyway, I'm happy for now. Many thanks, Jon.
And I'm very grateful to other helpful people in this thread too. MobileRead rocks!