i'm having the same problem here and a long press does not work. sometimes it even crashes koreader (!), not sure what's up with that.
i tried with "Cool reader" and "mupdf", neither work.
This is a regression in the 2021.07 release too, i downgraded to 2021.05 and the books load fine.
nevermind that: reinstalling 2021.07 fixes the issue completely, probably a fluke in the original install, see also
https://github.com/koreader/koreader/issues/7998