Leo Wagner: I had the same issue. I suppose it's because now white's group has only one liberty and can instantly be killed.
caranthir: Yeah, it seems that the problem has been modified. If there were an extra liberty, then I'd usually prefer Fergalicious's sequence, since the remaining ko threat (if not ate-connect-take first) is bigger for the attacker.
Fergalicious: Sequence wC1,bD1,wC2,bF1,wA1 gives the same result but removes a ko threat which could be used by either player. I may be wrong, but is this not a successful result also?
I had the same issue. I suppose it's because now white's group has only one liberty and can instantly be killed.