I've reached the same position as s_pugh, so I don't think that there's an error. I see that the preamble begins "In 35 answers one 'wrong' letter must be replaced by a 'right' letter", so as long as the replacement letters in the six clues where the replacement letter is unchecked are "right", and the full set of "wrong"/"right" combinations is used up, the solution would have to be considered correct. But I'm uneasy at the possibility of four different correct grids.