Greetings,
I am having the same problem on a Sigma SG-50B. I am suspecting the coin optics with the "N" on it. I get about 80% to feed correctly, but the others that give me the alarm cause me to have to use the key and reset the error. I have cleaned the optics as the book says and still have this problem. I have the sensitivity set very loose on the CC, but still get these errors. Checked all the plugs and disconnected-reconnected them, but still have the error.
Maybe some one else has seen this problem and can help us out.