Arduino reads bogus from serial

Recently I've started playing around with an Arduino Uno (rev 3), and I've set up a communication bridge between the Arduino and my computer using Python. So I send a command in Python ("ping" for instance), which is then read and interpreted. I've had it working before, using the Python commands to control a motor, but today it became non-responsive. However, only if I boot the serial monitor from the Arduino IDE, it suddenly accepts my commands again. Further inspection reveals that the Arduino is probably reading empty or corrupted data, interpreting it as ð, à and ÿ and such.

Here is an excerpt of the Python code I'm using:

import sys, glob, serial, time, re

class interface:

     timeout = 2    # 10 second time-out

    def __init__(self):
        port = "/dev/ttyACM0"
        self.connect(port)
        pass

    def connect(self, port="/dev/ttyACM0"):
        global arduino
        arduino = serial.Serial(port, baudrate=115200, bytesize=8, parity=serial.PARITY_NONE, stopbits=1, timeout=0.1)  # Establishing connection
        time.sleep(3) # Allowing the connection to settle
        arduino.flush()
        print "Connection established..."

    def ping(self):
        arduino.flush()
        arduino.write("ping")
        msg = self.listen_arduino()
        print msg
        if msg == "ping_ok":
            print "pinged"
            return True
        return False

    def listen_arduino(self):
        timeout = self.timeout
        matcher = re.compile("\n")
        buff = ""
        t0 = time.time()
        buff += arduino.read(128)

        while (time.time() - t0) < timeout and (not matcher.search(buff)):
            buff += arduino.read(128)
        return buff.strip()

And here is the Arduino C code:

String readString;

void setup() {
    Serial.begin(115200); // use the same baud-rate as the python side
}

void loop() {
    readString = listen(); // listen for any commands
    if (readString.length() > 0) {

        Serial.println(readString); // print the command

        if(readString == "ping") {
          Serial.write("ping_ok");
        }
    }
}

String listen() {
    String readString;
    while (!Serial.available());
    while (Serial.available()) {
        delay(1); // Allow the buffer to fill
        if (Serial.available() > 0) {
            char c = Serial.read();
            readString += c;
        }
    }
    return readString;
}

I suspect there is something fishy with the connection I'm making on the Python side, as the Arduino IDE serial monitor works just fine. I hope anyone can spot the problem in this code, I'm willing to provide more details on request.

Thanks