[davidfor]

Originally Posted by anacreon

I realized I had tried to convert azw to kepub, so I tried azw to epub and the conversion worked. When I tried to edit the resulting epub, the editor told me the epub had unrecognized fonts and the "clé de reconnaissance" (something like recognizing key in English?) was missing from the OPF, and I should convert epub to epub before editing. It worked.

Then I went to Sigil, which said "Sigil has created a template NCX to support epub2 backwards compatibility."

The resulting clean epub3 converted to kepub without apparent difficulty.

But when I tried opening the resulting kepub from Calibre, I had the following error message

Spoiler:

calibre, version 4.23.0
ERREUR : Le chargement du livre a échoué: Impossible d'ouvrir le livre à la position C:\Programmes Portables\Calibre Port\Calibre Library\Wolff, Larry\The Boys and Their Baby (21169)\The Boys and Their Baby - Wolff, Larry.kepub. Cliquez sur "Afficher les détails" pour plus d'informations.

Traceback (most recent call last):
File "site-packages\calibre\gui2\viewer\ui.py", line 468, in _load_ebook_worker
File "site-packages\calibre\gui2\viewer\convert_book.py", line 234, in prepare_book
File "site-packages\calibre\gui2\viewer\convert_book.py", line 191, in do_convert
ConversionFailure: Failed to convert book: C:\Programmes Portables\Calibre Port\Calibre Library\Wolff, Larry\The Boys and Their Baby (21169)\The Boys and Their Baby - Wolff, Larry.kepub with error:
C:\Programmes Portables\Calibre Port\Calibre\\app\pylib.zip\dateutil\parser\_parse r.py:1177: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
Failed to run pipe worker with command: from calibre.srv.render_book import viewer_main; viewer_main()
Python function terminated unexpectedly
No plugin to handle input format: kepub (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 114, in main
File "site.py", line 88, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 189, in main
File "<string>", line 1, in <module>
File "site-packages\calibre\srv\render_book.py", line 862, in viewer_main
File "site-packages\calibre\srv\render_book.py", line 855, in render_for_viewer
File "site-packages\calibre\srv\render_book.py", line 830, in render
File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 54, in extract_book
File "site-packages\calibre\ebooks\conversion\plumber.py", line 755, in __init__
ValueError: No plugin to handle input format: kepub

This is a plain epub, which I can read as an epub, so I don't plan to report the bug, unless the epub isn't readable on my Kobos: it seems to be just a very badly made azw - but I can hardly complain to Amazon that I can't convert their book...

But, it isn't a plain epub. You stated you converted it to kepub. Calibre is trying to open that kepub. As @theducks said, you probably need to install the Kepub Input plugin, but, at one time calibre was treating kepubs as epubs when using the viewer. It has been a while since I tried that, so I don't know when it changed.

But, if the kepub has problems on the Kobo device, I suggest converting the AZW to epub and fixing that using the editor. Then convert that to kepub and sending it to the device. Or use the KoboTouchExtended driver.