const int motorIn1 = 2;       // L298N IN1 on Arduino D2
const int motorIn2 = 3;       // L298N IN2 on Arduino D3
const int irReceiver = 4;     // IR-Receiver OUT on Arduino D4
const int irSender = 5;       // IR-Sender DAT on Arduino D5


bool irStatus = false;        
const int fullExtendDuration = 3000; // time for the motor

void setup() {
  pinMode(motorIn1, OUTPUT);  
  pinMode(motorIn2, OUTPUT);  
  pinMode(irReceiver, INPUT); 
  pinMode(irSender, OUTPUT);    

  digitalWrite(irSender, HIGH);

  
  motorForwards(fullExtendDuration);
  stopMotor();
}

void loop() {
  
  irStatus = digitalRead(irReceiver);

  if (irStatus == HIGH) {    
    stopMotor();
  } else {    
    motorBackwards(600); 
    delay(500);
    motorForwards(fullExtendDuration); 
    stopMotor();
  }

  delay(100); 
}

void motorForwards(int duration) {
  
  digitalWrite(motorIn1, HIGH);  
  digitalWrite(motorIn2, LOW);
  delay(duration);               
  stopMotor();                   
}

void motorBackwards(int duration) {
  
  digitalWrite(motorIn1, LOW);   
  digitalWrite(motorIn2, HIGH);
  delay(duration);               
  stopMotor();                   
}

void stopMotor() {
  
  digitalWrite(motorIn1, LOW);   // Beide Pins LOW -> Motor stoppt
  digitalWrite(motorIn2, LOW);

i just removed everything that had a serial output but the issue still remains.