The problem is that it is basically an invalid epub file as its internal structure is incorrect.
A way I have found to fix such books is to
- Change the file extension to ZIP and extract the contained files
- Locate the html files that contain the text of the book, and then create an index.html file that points to them in the right order
- Add the index.html file to Calibre to create an HTML version of the book
- Use Calibre to convert the freshly imported file to epub, which now has the correct internal structure for an epub