MQTT - ESP can't connect anymore, MQTT Explorer can

Configuration
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1

Dear Community,

I have an ESP (D1 Mini) which connects to my MQTT Mosquito broker.
The thing worked. I did not change anything. Now it does not work anymore.

It can not connect to the Broker, client.state() gives back error -2 which stands for MQTT_CONNECTION_REFUSED_BAD_USERNAME_OR_PASSWORD.

The same credentials are working in MQTT Explorer. (And they were working on the D1 Mini as well)

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connects flawlessly.
I see all things published by my PCs sensors and so on. Only the ESPs do not connect, so they are not listed.

The minimal example ESP code

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

// Wi-Fi network credentials

// MQTT broker settings
const char* mqtt_server = "homeassistant";
const int mqtt_port = 1883;
const char* mqtt_username = "mqtt";
const char* mqtt_password = "Dreierpack1!";

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  Serial.begin(115200);

  // Connect to Wi-Fi
  connectToWiFi();

  // Connect to MQTT broker
  client.setServer(mqtt_server, mqtt_port);
  connectToMQTTBroker();
}

void loop() {
  if (!client.connected()) {
    connectToMQTTBroker();
  }
  client.loop();
}

void connectToWiFi() {
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println("");
  Serial.println("Wi-Fi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
}

void connectToMQTTBroker() {
  while (!client.connected()) {
    Serial.print("Connecting to MQTT broker...");

    if (client.connect("WemosD1Mini", mqtt_username, mqtt_password)) {
      Serial.println("Connected");
    } else {
      Serial.print("Failed, rc=");
      Serial.print(client.state());
      Serial.println(" Trying again in 5 seconds");
      delay(5000);
    }
  }
}

Connects to the Wifi and delivers the Eroor code -2 when trying to connect to MQTT.

Why so?

2

Edit:

All of my MQTT devices are affected by this. I have another ESP that also subscribed to the mosquito broker. It is offline as well.

3

How does your ESP devices turn homeassistant in

const char* mqtt_server = “homeassistant”;

… into an IP address?