Book cover mystery

Apologies if this topic is covered elsewhere or if I'm posting to the wrong place. This is my first post-introduction post.

Using Calibre I converted a book yesterday from .ODT (open office) to Epub and Mobi. The books appear to be properly formatted, yay. I used the metadata editor to specify a cover, and that shows up in the Calibre pane.

But when I open the book to read it in the ebook reader, the cover is wrong. It just shows a glyph (my logo) blown up to fill the cover space. Not sure where it's grabbing that logo from, but it's wrong, obviously. Somehow the reader isn't finding the cover.

When I open the book up and click on the metadata.opf file I get a failure, with the following message:

calibre, version 2.60.0
ERROR: Could not open ebook: Failed to read book, /Users/johnsundman/Dropbox/Calibre Library/John Sundman/Biodigital - free version for mail list subscribers (31)/metadata.opf click "Show Details" for more information

Traceback (most recent call last):
File "site-packages/calibre/utils/ipc/simple_worker.py", line 274, in main
File "site-packages/calibre/ebooks/oeb/iterator/book.py", line 62, in extract_book
File "site-packages/calibre/customize/conversion.py", line 241, in __call__
File "site-packages/calibre/ebooks/conversion/plugins/html_input.py", line 83, in convert
File "site-packages/calibre/ebooks/conversion/plumber.py", line 1258, in create_oebbook
File "site-packages/calibre/ebooks/oeb/reader.py", line 71, in __call__
File "site-packages/calibre/ebooks/oeb/reader.py", line 696, in _all_from_opf
File "site-packages/calibre/ebooks/oeb/reader.py", line 343, in _spine_from_opf
OEBError: Spine is empty

Any guidance appreciated.

Thanks!

jrs