Yes your OPF file does not meet epub2 spec:
(
http://idpf.org/epub/20/spec/OPF_2.0...htm#Section2.4)
The manifest appears to be fine as it lists the exact NCX file with the proper href and proper media type.
The problem is according to the epub2 spec, the spine must have a toc attribute that must point to the manifest id of the NCX as the NCX is required in epub2.
The manifest id on your ncx file is:
id="ncx"
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:
to
And then save it. You should no longer get that warning or error message.
KevinH