After some searching on the Web, I find that this is the most common problem with generating the epub file.
I have tried several solutions found here and there and have at last found one that actually works, resulting in a validation from
http://threepress.org/document/epub-validate/
The following zip commands did the trick:
zip -q0X ROE.epub mimetype
zip -qXr9D ROE.epub *
Hope this will help others.