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I downloaded Abiword 2.9.1 and looked at the opening readme file. I saved it as ePub and got very close to a working ePub file. The content.opf was missing the full list of items in the manifest which is required. That section should read:

   <manifest>
        <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
        <item id="index.xhtml_files_style.css" href="Styles/index.xhtml_files_style.css" media-type="text/css"/>
        <item id="index.xhtml" href="Text/index.xhtml" media-type="application/xhtml+xml"/>
    </manifest>

The spine item is shown as indexxhtml and should be index.xhtml. It is missing the decimal point. Adding the two entries plus the decimal point provides an working file that can be opened in Adobe Digital Editions as a starting point for improvements.