How to create a simple TCP server with the analogRead value on Wemos D1 mini?
So the problem was that I forgot the following line.
Thanks to @Johnny Mopp for pointing this out.

server.begin()


Correct code:

#include &lt;ESP8266WiFi.h&gt;

//how many clients should be able to telnet to this ESP8266
#define MAX_SRV_CLIENTS 2
const char* ssid = "myWifi";
const char* password = "myPassword";

WiFiServer server(23);
WiFiClient serverClients[MAX_SRV_CLIENTS];

unsigned long previousMillis = 0;
const long interval = 200;
void setup() {
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  uint8_t i = 0;
  while (WiFi.status() != WL_CONNECTED &amp;&amp; i++ &lt; 20) delay(500);
  if (i == 21) {
    Serial.print("Could not connect to"); Serial.println(ssid);
    while (1) delay(500);
  }
  server.begin();
  server.setNoDelay(true);
}

void loop() {
  unsigned long currentMillis = millis();
  uint8_t i;
  //check if there are any new clients
  if (server.hasClient()) {
    for (i = 0; i &lt; MAX_SRV_CLIENTS; i++) {
      //find free/disconnected spot
      if (!serverClients[i] || !serverClients[i].connected()) {
        if (serverClients[i]) serverClients[i].stop();
        serverClients[i] = server.available();
        continue;
      }
    }
    //no free/disconnected spot so reject
    WiFiClient serverClient = server.available();
    serverClient.stop();
  }

  //do every 200ms
  if (currentMillis - previousMillis &gt;= interval) {
    previousMillis = currentMillis;

    int isensorValue = analogRead(A0);
    String sSensorValue = String(isensorValue) + "\n\r";
    size_t len = sSensorValue.length();
    uint8_t sbuf[len];
    sSensorValue.getBytes(sbuf, len);
    for (i = 0; i &lt; MAX_SRV_CLIENTS; i++) {
      if (serverClients[i] &amp;&amp; serverClients[i].connected()) {
        serverClients[i].write(sbuf, len);
        delay(1);
      }
    }

  }
}