testESP8266.ino

#include <SoftwareSerial.h>

// ESP8266 rx, tx
SoftwareSerial esp8266(11, 12);

void setup() {
	//connection to ESP8266
	esp8266.begin(9600);
	esp8266.setTimeout(2000);
	//serial debug
	Serial.begin(9600);

	// enable wifi
	pinMode(13, OUTPUT);
	digitalWrite(13, HIGH);
	delay(1000);

	if (check()) {
		resetESP8266();

		if (connectWifi("ssid", "password")) {
			String response="";
			boolean ok = httpPOST("test=42", "192.168.1.24", 3000, "/event", &response);
			if (ok) {
				Serial.println(response);
			}
			else {
				Serial.println("Something went wrong while posting data");
			}
		}
	}
}

void loop() {
}

boolean check() {
	esp8266.println("AT");
	Serial.println("checking..");
	boolean ok = false;
	if (esp8266.find("OK")) {
		Serial.println("ESP8266 available");
		ok = true;
	}
	return ok;
}

void resetESP8266() {
	// reset ESP8266
	esp8266.println("AT+RST");
	delay(3000);
	// set station mode
	esp8266.println("AT+CWMODE=1");
	delay(300);
	// reset ESP8266
	esp8266.println("AT+RST");
	delay(500);
	esp8266.println("AT+CIPMUX=0");
	delay(500);
}

boolean connectWifi(String ssid, String password) {
	String cmd = "AT+CWJAP=\"" + ssid + "\",\"" + password + "\"";
	esp8266.println(cmd);
	boolean connected = false;
	for(int i = 0; i < 20; i++) {
		Serial.print(".");
		if(esp8266.find("OK")) {
			Serial.println("Connected!");
			connected = true;
			break;
		}
		delay(50);
	}
	if(!connected) {
		Serial.println("Cannot connect to wifi");
	}
	return connected;
}

boolean httpPOST(String data, String server, int port, String uri, String* response) {
	// initiate TCP connection
	String start = "AT+CIPSTART=\"TCP\",\"" + server + "\"," + port;

	// prepare the data to be posted
	String postRequest =
		"POST " + uri + " HTTP/1.1\r\n" +
		"Host: " + server + ":" + port + "\r\n" +
		"Accept: *" + "/" + "*\r\n" +
		"Content-Length: " + data.length() + "\r\n" +
		"Content-Type: application/x-www-form-urlencoded\r\n" +
		"\r\n" +
		data;

	// notify ESP8266 about the lenght of TCP packet
	String sendCmd = "AT+CIPSEND=" + postRequest.length();


	esp8266.println(start);
	if (esp8266.find("OK")) {
		Serial.println("TCP connection OK");
		esp8266.println(sendCmd);
		if (esp8266.find("SEND OK")) {
			Serial.println("Packet sent");
			while (esp8266.available()) {
				String tmpResp = esp8266.readString();
				response = &tmpResp;
			}
			// close the connection
			esp8266.println("AT+CIPCLOSE");
			return true;
		}
		else {
			Serial.println("Cannot send packet");
			return false;
		}
	}
	else {
		Serial.println("Cannot initiate TCP connection");
		return false;
	}
}

The line 87 should change to:

        "Accept: *" +
        "/" +
        "*\r\n" +

Arduino IDE think it's a comment mark.

I added these lines after line 57 then everything works.

    esp8266.println("AT+CIPMUX=0");
    delay(500);

Great! I'll try this asap. But i think i bricked a cactus micro... I'll try with my other one and I'll ask you for my brick problem.
Thanks for investigating!

So I tried and still cannot initiate the TCP connection.
I never receive the "OK" from line 100