[FEN ""] 1. Nh3 f5 2. e4 fxe4 3. d3 exd3 4. Be2 Kf7 5. Qd2 dxe2 6. O-O
It's letting the white king castle through a square that is attacked by a black pawn; that is an illegal move!
Do you have any idea why this is happening?
EDIT: I’m doing this here since comments can’t include pictures. In regards to what you said Brian Towers, you actually are able to use Apronus to validate games. Take a look at this.
That’s what happens when you put in an illegal move for a position and tap the “Absorb FEN Or PGN Button.”
EDIT 2: I forgot to include this proof link of my claim.