Serial communication error , can not figure it out. Help !!

Environment : UDOO quad.
The J18 jumper is not used( empty ).
CN6 is not connected, ie I am using direct serial connection between iMX6 with SAM arduino.
Potentiometer is connected to A0 pin on arduino.

I am trying to send data from analog reading on the Arduino side via serial comm to iMX6.
I can see the data being generated and print on the arduiono monitor.
When I run Python code, I can see data on python side for about 8-10 lines then the error. ( see below )

Code:

// Adruino code 
int ADCpin0 = A0;
void setup() {
  Serial.begin( 9600);
}

void loop() {
  int myVal = analogRead(ADCpin0);
  Serial.println( myVal, DEC);
  delay(100);
}

# this is my Udoo linux python code( iMX6 side )
# reading serial data

Code:

#!/usr/bin/python
import serial 
ser = serial.Serial(timeout=1);
ser.port = "/dev/ttymxc3"
ser.baudrate = 9600 
ser.open()
data = ""

try : 
	while True:
		ch = ser.read()    # **** 
		while ch != '\n':
			data = data + ch
			ch = ser.read()
		
		print "Data: " + data
		data = ""

except Exception, e : 
		print "error ::" + str(e )

Result on python screen

Code:

Data: 167
Data: 166
Data: 12
error ::device reports readiness to read but 
returned no data (device disconnected or multiple access on port?)

I also tried
ser.readline() #****
It gave the same error.

Question : what is causing this error mesage ?
Thanks.
samira