Gardenpump problem - problem solved! Sorry - logic mistake: need to use long int or unsigned int

I try to activate a garden-waterpump for a fountain in short intervals, i.e. 10sec ON, 50sec OFF. The code is very simple, and it works on my desktop with a lamp connected - as I want it to.
When installing it in my gardenhouse (dry environment ) it stops working - sometimes right away -sometimes after a while.
I use a standard water pump 220V from “Bauhaus”. I tried to connect it through a double relay (for both N and P), and have grounded it as prescribed.
The relay is powered by an external power supply, so there shouldn´t be any galvanic connection between the 220V circuit and the Arduino. (I used a double identical setup - one with an Arduino Nano, the other with an Arduino Uno). Exactly same result…

I guess there must be some kind of an interference from the switching in the relay with the Arduino… Any ideas???

#include <Wire.h>


int relayPin = 6; //pin 6 går til LED som test eller senere til relais vor 220V
int startPin=7;
int stopPin=8;


//indstilling af antal cykler per time - og vandingstid per gang i sec.
int antalCyklerT =600; // number of cycles per hour ... her gives antal perioder som der vandes her time - fra 1-30 per time
int vandingTidSec = 1; //on-time in seconds ... her indstilles hvor mange sekunder pumpen skal køre ad gangen.
int vandingPauseSec = ((3600 / antalCyklerT) - vandingTidSec);//break between ON times (calculated value)

int n=1;

void setup () {

  Serial.begin(9600);
  delay(3000);
  pinMode(relayPin, OUTPUT);
  pinMode(startPin, OUTPUT);
  pinMode(stopPin, OUTPUT);
  Wire.begin();


  Serial.print("Number of cycles: " );
  Serial.print(antalCyklerT);
  Serial.print(" - per hour");
  Serial.println();
  Serial.println();
  Serial.print("ON time in seconds: " );
  Serial.println(vandingTidSec);
  Serial.print("OFF time in secondes: " );
  Serial.println(vandingPauseSec);
  Serial.println();


    digitalWrite(startPin, LOW);
    digitalWrite(stopPin, HIGH);
    digitalWrite(relayPin, HIGH);

n=1;
Serial.print("N:=");
Serial.println(n);
delay(1000);
}

void loop () {
while (n>=1){ //the loop is running indefinitly - for testing purpose

    digitalWrite(relayPin, LOW);
    digitalWrite(startPin, HIGH); 
    digitalWrite(stopPin, LOW);



    delay (vandingTidSec * 1000); //delay for antal sekunder

    digitalWrite(startPin, LOW);
    digitalWrite(stopPin, HIGH);
    digitalWrite(relayPin, HIGH);

 
    
    delay(vandingPauseSec * 1000);
    n=n+1;
    Serial.print("N:=");
    Serial.println(n);
}
  }

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