After your 2 messages, I decided to test again from scratch by installing Calibre (+KoboTouchExtended plugin) and kepubify on the laptop of my daughter who owns the e-reader.
This time the kepub conversion with Calibre was a success, as was the transfer. In the end, the book is perfectly recognized and the pages are correctly displayed.
With kepubify v3.1.6 the conversion was much faster. Unfortunately the result is not conclusive: the same problem of displaying blank pages except the cover which is correctly displayed this time.
So I have partially solved my issue, with the Calibre workaround acceptable, although of course I much prefer to use kepubify .
Finally, you might be interested to have a look at my kepubified VictorHugo and compare it with yours. I wonder how it will look on your e-reader (check your inbox).