Maximum number of client.println() headers?

Hi, I am sending POST request from esp32 to my heroku server. Everything was working just fine until I added one more header field to request. After that server did not response. From postman server accepted the request so there is something bad in esp. Do you have experience with something like this?

  if (!client.connect(SERVER, 443)) {
    Serial.println("connection failed");
    return 522; //HTTP status code: SSL Handshake Failed
  }
  else {
    Serial.println("connection successful\n");
    userData PostData;
    PostData.fillUser(cardUID, action, type, from, date);
    client.println("POST https://"SERVER PATH1" HTTP/1.1");
    client.println("Host: "SERVER);
    client.println("Cache-Control: no-cache");
    client.println("Connection: close");
    client.println("Content-Type: application/json");
    client.print("Content-Length: ");;
    client.println((PostData.stringify()).length());
    client.println("mcu_key: " MCU_KEY); //after I added this never got response
    client.println();
    client.println(PostData.stringify());

client.println("mcu_key: " MCU_KEY); //after I added this never got response

Does it work if you send the data properly, with two print statements:

    client.print("mcu_key: ");
    client.println(MCU_KEY);

@PaulS Thanks for answer, I tried it but still the same results.