Hi Doitsu,
I was able to workaround the issue by unpacking the epub (it's just a zip file) and using Visual Code I replaced all occurrences in all the files of "display:none" to "display:hidden". Zipping back the files and directories into epub opened the book exactly as before.
Now kindlegen succeeded!
So, if anyone hits the same problem can try applying the above. I don't know what's the difference between "display:none" and "display:hidden". But it seems to work.
It looks right and the pictures are not elongated as when using Calibre.
I now have only one remaining problem. Some characters appear in my kindle book. I think kindlegen used my active code page and not UTF8.
I will try KindlePreviewer which is using a newer version of kindlegen.
Thanks,
Codrut