That would tie in with the board being in permanent transmit mode due to /RE & DE being pulled high (according to the schematic). I must be missing something very obvious because controlling the direction of a half-duplex RS485 transceiver is such a basic thing, whether done automatically in electronics or manually through RE & DE pins.
Hopefully some other forum members like @pylon with more Modbus knowledge can shed some light on this issue and point out where we're going wrong, because it shouldn't be this hard to get RS485 working!