Python break loop and go into loop again

Because your readResponse() function breaks if and only if it encounters a serialTimeoutException. Note that you've put the break statement under the if block which in turn is inside the except block. This means as long as your program doesn't encounter that exception the while loop goes on infinitely. If you want the function to return immediately after it executes the try block inside readResponse(), then just put a return statement right after print(ser.readline()) and put the time.sleep() method at the end of the readResponse() function. Don't nest it inside the if block.

P.S. The time.sleep(1) here:

if (time.time() > timeout):
        break
        time.sleep(1)

is dead code. I think you need to rethink what you wanted to do with the if block.

I've found the solution myself

import serial
import time
ser = serial.Serial('COM10', 9600, timeout=0)
timeout = 60
timeout_start = time.time()

def readresponse():

while time.time() < timeout_start + timeout:
    try:
        print(ser.readline())
        time.sleep(1)
    except ser.SerialTimeoutException:
        print('Data could not be read')
    time.sleep(1)


def sendHttpPost():
    ser.write('AT+HTTPACTION=1\r'.encode())

def initialAtCommands():
    ser.write('AT+SAPBR=3,1,\"Contype\",\"GPRS\"\r'.encode())
    ser.write('AT+SAPBR=3,1,\"APN\",\"APN\"\r'.encode())
    ser.write('AT+SAPBR=1,1\r'.encode())
    ser.write('AT+HTTPINIT\r'.encode())
    ser.write('AT+HTTPPARA=\"CID\",1\r'.encode())
    ser.write('AT+HTTPPARA=\"URL\",\"URL\"\r'.encode())

if(ser.isOpen() == False):
    print("Serialport is currently not connected")
else:
    print("Serialport successfully connected")
    initialAtCommands()

while 1:
    sendHttpPost()
    time.sleep(1)
    readresponse()
    timeout_start = time.time()
    time.sleep(1)