• Install ArduinoISP program to ProMini (Programmer)
• Connect MySensors board on ProMini (ArduinoISP) via ISP header
• Select, Programmer="Arduino as ISP", Board:"Sensbender Micro", Processor:"Atmega 328 8Mhz"
• Now click on "Burn Bootloader". bootloader will be loaded on MySensors board
• Remove ISP header and remove ProMini from your computer
• Connect MySensors board to USB via FTDI connector
• Upload MySensors sketch(OTA should be enabled on the sketch) to MySensors board
• Now the MySensors node will appear in MyController server.
• Modify the MySensors sketch and upload it to MyController
• Now trigger "Firmware upload" from MyController server page.

This is the first sketch to upload one time with the USB to FTDI connector :

//pr1: minimal sketch for fota with blink led


#define blinkTime 200  
#define SKETCH_NAME "SensBender Led 200 -70"  //25 caracteres max
#define SKETCH_VERSION "pr1"

//===== RFM config =====
// Enable and select radio type attached
#define   MY_RADIO_RFM69
//#define   MY_DEBUG_VERBOSE_RFM69
//#define   MY_DEBUG_VERBOSE_RFM69_REGISTERS

#define   MY_RFM69_NEW_DRIVER
//#define MY_RFM69_ATC_MODE_DISABLED
#define MY_RFM69_ATC_TARGET_RSSI_DBM (-70)  // target RSSI -70dBm//BUG
//#define MY_RFM69_MAX_POWER_LEVEL_DBM (20)   // max. TX power 10dBm = 10mW//MARCHE
#define   MY_RFM69_FREQUENCY (RFM69_868MHZ)
#define   MY_IS_RFM69HW //with H(CW)
//#define   MY_RFM69HW (true)
//#define   MY_RFM69_NETWORKID (100)

//#define MY_RFM69_BITRATE_MSB (RFM69_BITRATEMSB_1200)
//#define MY_RFM69_BITRATE_LSB (RFM69_BITRATELSB_1200)

// Enable repeater functionality for this node
//#define MY_REPEATER_FEATURE

#define MY_DEBUG

//===== LED =====
#define LED_BUILTIN 9       //pb1 pin15

//===== This Node ID =====
#define MY_NODE_ID 0x02

//===== Enable OTA feature =====
#define MY_OTA_FIRMWARE_FEATURE
#define MY_OTA_FLASH_SS (8)       //pb0 pin14
#define MY_OTA_FLASH_JDECID 0x00  //0x00 if you don't know

#include <MySensors.h>

//===================================
//=              SETUP              =
//===================================
void setup() {//pas besoin de Serial.begin(115200)
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.print("Start "); Serial.print(SKETCH_NAME); Serial.print(" "); Serial.println(SKETCH_VERSION);
}//FIN du SETUP
//==========================================
//=              PRESENTATION              =
//==========================================
void presentation() {
  // Send the sketch version information to the gateway and controller
  sendSketchInfo(SKETCH_NAME, SKETCH_VERSION);
} // FIN de PRESENTATION
//===================================
//=              LOOP               =
//===================================
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);  Serial.print("LED ON ");
  wait(blinkTime);
  digitalWrite(LED_BUILTIN, LOW);  Serial.println("LED OFF ");
  wait(blinkTime);
}// FIN de LOOP

Change blinkTime time and compil the sketch , then upload firmware .hex with MYCONTROLLER.
Enjoy

also try #define MY_OTA_FLASH_JDECID 0x00, which works for me.

• I loaded the ArduinoISP sketch on an Arduino PRO mini to use as a programmer.
  

• then I configured Arduino IDE with the SensBender 8MHz board like this:
  
  and I started the bootloader burning which went well.

• I unplugged the ProMini and plugged in an FTDI converter to program the ATMega 328P by configuring like this:
  

I injected this simple BLINK LED sketch :

// Enable debug prints to serial monitor
//#define MY_DEBUG                  //debug de MySensors
//#define MY_DEBUG_VERBOSE_SIGNING  //debug de la signature

//===== RFM config =====
#define MY_RADIO_RFM69
#define MY_IS_RFM69HW //with H(CW)
#define MY_RFM69_NEW_DRIVER
#define MY_RFM69_FREQUENCY  RFM69_868MHZ
#define MY_SIGNAL_REPORT_ENABLED
//#define MY_RFM69_ATC_TARGET_RSSI_DBM (-80)
//#define MY_RFM69_MAX_POWER_LEVEL_DBM (10)   // max. TX power 10dBm = 10mW
//1mW = 0dBm  10mW = 10dBm  25mW = 14dBm  100mW = 20dBm

//#define MY_DEBUG_VERBOSE_RFM69  //debug du signal RF
//#define MY_RFM69_NETWORKID 101
#define MY_RFM69_IRQ_PIN 2
#define MY_RFM69_CS_PIN 10

//===== LED =====
#define LED_BUILTIN 9       //pb1 pin15

//========== NODE ID ==========
#define MY_NODE_ID 4
//#define MY_NODE_TYPE "NODE"
//#define CHILD_ID 1   // Id of the sensor child

#include <SPI.h>
#include <MySensors.h>
#include <Wire.h>

//pr1:===== FOTA =====
#define MY_OTA_FIRMWARE_FEATURE
#define MY_OTA_FLASH_SS (8)
#define MY_OTA_FLASH_JDECID (0xBF26)  //Use (0x00) if unknown.

//pr2: ===== tempo non bloquante =====
const unsigned long BLINK_INTERVAL = 1000;// Nombre de millisecondes entre deux changements d'état de la LED
unsigned long previousMillis = 0;// Précédente valeur de millis()
byte ledState = LOW;// Précédent état de la LED

//===================================
//=              BEFORE              =
//===================================
void before()
{

} //FIN de before

//===================================
//=              SETUP              =
//===================================
void setup() {//pas besoin de Serial.begin(115200)
  pinMode(LED_BUILTIN, OUTPUT);// Configure la broche de la LED en sortie
  digitalWrite(LED_BUILTIN, ledState);  // Configure l'état initial de la LED

}//FIN du SETUP
//==========================================
//=              PRESENTATION              =
//==========================================
void presentation() {
  // Send the sketch version information to the gateway and controller
  sendSketchInfo("led fota", "1.0");

} // FIN de PRESENTATION
//===================================
//=              LOOP               =
//===================================
void loop() {

  unsigned long currentMillis = millis(); // Récupére la valeur actuelle de millis()
  if (currentMillis - previousMillis >= BLINK_INTERVAL) // Si BLINK_INTERVAL ou plus millisecondes se sont écoulés
  { previousMillis = currentMillis;    // Garde en mémoire la valeur actuelle de millis()
    ledState = !ledState; // Inverse l'état de la LED
    digitalWrite(LED_BUILTIN, ledState); Serial.print("LED  "); Serial.println(ledState);
  }

}// FIN de LOOP

• the led is flashing well and MYController sees the node
  

• I change the flashing delay from 1000 to 500 and the version which goes from 1.0 to 1.1. I compile the sketch which gives me: moteino_MySensors_fota_pr1.ino.hex

• I go to Utilities>>Firmware and add a Type, Version 1.1 and Firmware (with faster flashing delay)
  

• I go back to Resources>>nodes and upload the firmware of this node n°4
  

• and ...nothing after 7 minutes
  

Any Idea ? My Sketch is wrong ?

• setup Arduino IDE for Sensebender - https://github.com/mysensors/ArduinoBoards
• Select your hardware details. from your input, the board: sensbender, 328 + 8 Mhz
• Burn bootloader from IDE (it should be DualOptiboot)
• add a sketch with #define MY_OTA_FIRMWARE_FEATURE - https://github.com/mysensors/MySensors/blob/2.3.2/MyConfig.h#L1222~L1253

Afterburn DualOptiboot, you should add a MySensors sketch via serial (OTA feature should be enabled on your sketch). From this point, OTA works with DualOptiboot