I had this happen with a book just the other day. I bought it from Amazon maybe 7-8 years ago, had Calibre convert it and read it om my Aura HD.
But now, just the cover.
epubcheck gave some errors (a "style" used in html-header). I fixed all those. No change.
But when I had Sigil recreate the toc, it worked.
Something must have changed in Kobo firmware since it used to work.
(kepub, as an epub it worked)
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