Make sure the encoder is still in between the encoder sensor all the way across the machine. If the sensor is not straddling the encoder, it can't read it. If that looks good, I would start by changing the sensor out. This error specifically means that the main board didn't receive a value from the encoder sensor. That means it's not reading at all which is why I point to the sensor. It could end up being any of the following however, scan motor, servo board, main board, or slider board.