id name Stockfish 17.1
setoption name UCI_Chess960 value true
position fen qnrkrbbn/pppppppp/8/8/8/8/PPPPPPPP/QNRKRBBN w KQkq - 0 1 moves e2e4 e7e5 O-O-O
It appears stockfish 17.1 does not parse "O-O-O" here and just attempts to generate a move for white instead, which is marked by arena as illegal since it is black's move. I assume stockfish needs "d1c1" here instead of "O-O-O" and this is sort of an arena problem?