I have exactly the same problem and the same car C230 1997, it hapenned after and emergency braking, it stayed on and has been on for the last 3 month. You mentioned the overvoltage relay, but how do you test if it is OK?
Is there a way of resetting the light or errasing the error? I have replaced the brake pads and the rotors since the incident and cleaned the magentic sensor that reads the teeth on the wheel and nothing, there has to be an easy and simple way of solving this, please help, any ideas are welcome. Thanks.