[DiapDealer]

I don't think it's the problem here (since the same epub displays as expected on a different machine with the same version of Sigil), but remember that Epubcheck doesn't validate css. It wouldn't pick up any potential problems or errors in your css. But as I said, I don't think the epub's css is the problem here. @BeckyEbook's suggestion does fit the symptoms, though. The Inspector would go a long way toward showing what (if any) css is being applied.