What errors do you see if you open the ePub in calibre's e-book editor? I would suggest installing the epubcheck plugin for the editor and using that as well as the built-in bugcheck.
Alternately, save a backup copy of your epub and do an epub to epub conversion to see if that fixes your issues.
If you clicked on the link supplied by Kovid, it lists the steps needed to report conversion issues.
In this case, I would suggest that your "epub" is not a valid epub since no opf file is found. The path to the .opf file
must be specified in the container.xml file in the META-INF directory in the .zip container (an ePub is a renamed .zip file).
Sample container.xml file below (the path to the .opf file is bolded):
Code:
<?xml version="1.0" encoding="UTF-8"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
<rootfiles>
<rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/>
</rootfiles>
</container>
If the file was obtained from a retail store, can you please supply the title/author/ISBN?