So what you're basically saying is the problem has been addressed in Calibre 5.2 and that's the way to go. Unfortunately I'm still on Windows Vista so I can't install Calibre 5.2 yet. The problem is that I've made so many adaptations in Windows Vista and the software running on it that I'm rather – no, actually very – hesitant about upgrading to windows 10.
Thank you for your help.
I'm happy now to know the probable solution which is one more incentive to finally make the jump to Windows 10.
Still one more question: how can I tell if an epub file is really a kepub? I don't see anything in this epub file referring tot anything related to Kobo, not in the css file and not in any other file.