"Send payload error" in HTTP POST - Arduino ESP8266

Question

I'm trying to make a POST HTTP request via ESP8266 using Arduino IDE and this library, but for any reason I'm getting this error:

[HTTP] POST... failed, error: send payload failed

Wifi connect's correctly and header seems to be processed correctly in the library and the same request is working in POSTMAN.

By researching a little in the library, especifically in this file ESP8266HTTPClient.cpp, this section of code is where the error is trigged, and it's because the call _tcp->write(&payload[0], size) returns 0 while size is 128 (the correct lenght of the payload I'm sending).

// send Payload if needed
if(payload && size > 0) {
    if(_tcp->write(&payload[0], size) != size) {
        return returnError(HTTPC_ERROR_SEND_PAYLOAD_FAILED);
    }
}

In the variable payload I'm sending the following (I also checked this is correct in POSTMAN):

{\"mode\":\"sync\",\"messageType\":\"4a0f64bf6fc780e39a51\",\"messages\":[{\"Humidity\":90,\"Temperature\":50,\"Brightness\":60,\"timestamp\":1}]}

Here is my code in Arduino IDE:

#include <Arduino.h>
#define DEBUG_ESP_HTTP_CLIENT
#include <ESP8266WiFi.h>
#include <ESP8266WiFiMulti.h>

//#include <ESP8266HTTPClient.h>

#define USE_SERIAL Serial

#include "ESP8266HTTPClient2.h"

ESP8266WiFiMulti WiFiMulti;

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

    USE_SERIAL.println();

    for(uint8_t t = 4; t > 0; t--) {
        USE_SERIAL.printf("[SETUP] WAIT %d...\n", t);
        USE_SERIAL.flush();
        delay(1000);
    }

    WiFiMulti.addAP("xxxxxxxxxxxx", "yyyyyyyyyyyyyy");
}

void loop() {
 // wait for WiFi connection
    if((WiFiMulti.run() == WL_CONNECTED)) {

        HTTPClient http;

        USE_SERIAL.print("\n---------------------\n[HTTP] begin...\n");
        // configure traged server and url

        http.setReuse(true);
        http.begin("iotmmsp1941648609trial.hanatrial.ondemand.com",
                    443,
                    "/com.sap.iotservices.mms/v1/api/http/data/ab5b0ae0-9e2c-47e2-ac93-0deea935a588");

        http.setAuthorization("Bearer 77eb2493769bb566f5bc346f6d598e7");
        //http.addHeader("Authorization", "Bearer 77eb2493769bb566f5bc346f6d598e7");
        http.addHeader("Content-Type", "application/json;charset=utf-8");
        http.addHeader("cache-control", "no-cache");

        USE_SERIAL.print("[HTTP] POST...\n");
        // start connection and send HTTP header and body
        int httpCode = http.POST("{\"mode\":\"sync\",\"messageType\":\"4a0f64bf6fc780e39a51\",\"messages\":[{\"Humidity\":90,\"Temperature\":50,\"Brightness\":60,\"timestamp\":1}]}");

        // httpCode will be negative on error
        if(httpCode > 0) {
            // HTTP header has been send and Server response header has been handled
            USE_SERIAL.printf("[HTTP] POST... code: %d\n", httpCode);

            // file found at server
            if(httpCode == HTTP_CODE_OK) {
                String payload = http.getString();
                USE_SERIAL.println(payload);
            }
        } else {
            USE_SERIAL.printf("[HTTP] POST... failed, error: %s\n", http.errorToString(httpCode).c_str());
        }

        http.end();
    }

    delay(3000);
}

Question:

Why is that part of the code in ESP8266HTTPClient failing? Why it's not sending the payload correctly?