Originally Posted by georg3200
It's the line with the logo. Make sure, that it is at the very beginning of the sections. Make sure, that the file logo.png is in the same folder as content.opf.
The OPF spec states that the <spine/> can only contain OPS Content Documents -- basically just HTML, in most cases. ADE may render a PNG listed in the <spine/>, but there's no guarantee another EPUB reader will.