IO bit 24 is the 9th of 10 IOs on JP4. The first 8 of the 10 IOs on both JP4 and JP6 have 150 ohm pull down resistors. So IO 24 is very high impedance input (megohms). So maybe you need a termination resistor.
But yes you would normally connect the shield on to DC Ground at the KFLOP end of the cable and leave the other end open.