[snarkophilus]

Originally Posted by KevinH

Yes your OPF file does not meet epub2 spec:
(http://idpf.org/epub/20/spec/OPF_2.0...htm#Section2.4)

No suprise there

So open the epub in Calibre or Sigil and open the content.opf tab. If it has not already been fixed for you ...

Then change spine tag as from:

<spine>

to

<spine toc="ncx">

And then save it. You should no longer get that warning or error message.

"Hmmm." The OPF definitely doesn't have toc="ncx" before I start:

NUC 5667> unzip -p *.epub | grep -a '<spine'
        <spine>
NUC 5668>

but when I open the epub in Sigil, I get the same popup about but opening the OPF shows that (I assume) Sigil has changed the spine entry automatically to

  <spine toc="ncx">

If I try a bookmark I get the same error as before:

The file "OEBPS/OEBPS/Robi_9780553898279_epub_c01_r1.htm" does not exist.

Saving and re-opening the saved epub then indeed doesn't show an error, but the Sigil still thinks the bookmark references still all start with "OEBPS/OEBPS/..."

I'll send you a link to a scrambled version of the epub that shows the problem.