I could not test this problem with mayhemduck20 - it consistently crashes with
xboard: Error writing to first chess program: Broken pipe

I tested this with mayhemduck19, which also incorrectly flagged a move as illegal.

1.e4,e6 Nf6,c3
2.e5,d4 Ne4,e6
3.d3,g5 Nc5,c3
4.d4,a6 Ne6,c5
5.d5,d4 Nf4,d2
6.Qf3,e4 Ng6,f4
7. Qe4,d6 e6,c3
8. h4,f6 Bb4,c4+

My intended move was 9.h5, d2 but this was incorrectly flagged as illegal. There is no such thing as "check" for Duck chess.