Short answer. Bosch error 503 is a speed sensor fault. The system cannot read the wheel speed correctly, usually because the sensor is misaligned, the spoke magnet is missing, or the sensor cable is damaged.
Step by step
Check the spoke magnet. The small magnet on the rear spoke must be present and aligned with the sensor on the chainstay. Distance should be 5 to 10 millimeters.
Realign the sensor. The sensor mounts to the chainstay. If it has shifted, loosen and slide it closer to the magnet, then retighten.
Inspect the sensor cable. Follow the cable from the sensor up to the frame. Any pinch, cut, or corroded connector causes error 503.
Swap the sensor. Speed sensors are inexpensive and swap in about 15 minutes. If magnet alignment is right and the cable is good, the sensor itself is bad.