I have just tested the recipe and cannot reproduce your error (I even updated to your calibre version, just in case).
From the error log, the problem has to do with the 'browser.back()' statement just before downloading the audio files. Seems like a previous download failed (which should be the pdf file, if enabled).
Did you try to download the files manually from the web page? There might just be some temporally connection problem between your provider and the ZEIT server.
Hope this helps
PS: Just a note to help make the forum thread more readable: You should enclose larger code-snippets (like the error log) within '[ CODE][ SPOILER] [/SPOILER][/CODE]' marks (the '#' symbol and the one with the X'd eye).