it's not quite clear what is meant by the 'substitution key' here.
you get the polybius square, which sometimes is just randomly filled with the characters and the whole square is the key, and sometimes begins with a keyword and is filled with the remaining characters.
i have a perfectly working solution with a logical looking polybius square containing a german 6-character-word, but it tells me it's wrong and i can't find any other solution :(
//edit:
ok, after hours and hours of confusion i finally figured out that there are two solutions which yield an identical but transposed polybius square, but of course have different transposition keys and i tried the wrong one all the time :-/