If the book was already on the device, DSNB is probably write. But, this behaviour will also happen if the TOC in the book is invalid. If the pointers in the TOC do not actually point to files and anchors in the book, any attempt to use them will fail. You can probably see this using the calibre viewer, but it is a bit more forgiving for errors than the Kobo ereaders.
The fix is probably to removed it from the device and regenerate the TOC using either calibre or Sigil. Then resend it to the device.
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