pubsubclient: KeepaliveTimeout error with different brokers

When I connect to the MQTT server it works, but after 10 - 30 seconds the broker gives a keepalive timeout and reconnects rightaway. The loop is continuously executed. I tried different brokers but still the same problem. Mosquitto and Aedes (Node-Red) both are set to 60 seconds by default. So the broker expects every 60 seconds a PINGREQ. The esp8266 are default set to 15 seconds in the PubSubClient Library. So every 15 seconds it sends a PINGREQ. The code is without any delay so client.loop() is constantly being updated. Why do I still get all these disconnections?

void loop(){
 reconnectWifi();
 client.loop();
}

void reconnectWifi() { //(re)connect to WiFi and MQTT Server.
  if (WiFi.status() != WL_CONNECTED && currentMillis - wifiLostTimer >= 500UL ) {
    wifiLostTimer = currentMillis;
    delay(0); //to keep wifi connection alive same als yield()
    Serial.println("Wifi connecting");
    wifiLost = true;
    mqttLost = true;
    reconnectTried++;
    if (reconnectTried == 60 && digitalRead(relaySwitch) == relayOffOutput) {
      ESP.restart();
    }
  }

  else if (WiFi.status() == WL_CONNECTED && wifiLost == true) {
    reconnectTried = 0;
    wifiLost = false;
    Serial.println("Wifi Connected!!!");
  }
  // Loop until we're reconnected
  else if (client.connected() == false && currentMillis - timeOut >= 10000UL && WiFi.status() == WL_CONNECTED) {
    mqttLost = true;
    delay(0); //to keep wifi-connection alive
    timeOut = currentMillis;
    Serial.println("Attempting MQTT connection...");
    // Attempt to connect
    Serial.println(deviceID.c_str());
    client.connect(deviceID.c_str());
  }
  else if (client.connected() == true && wifiLost == false && mqttLost == true) {
    mqttLost = false;
    Serial.println("MQTT Connected!!!");
    String rxTopic = rxTopicFirst + deviceID;
    client.subscribe(rxTopic.c_str());
    Serial.println("Subscribed to: " + rxTopic);
  }
}