UART RX ambiguities

Hello,

i'm trying to use UART with STM32F767ZI Nucleo board. I'm using the following simple code (the essential parts):

/* USER CODE BEGIN PV */
uint8_t a[] = {0x81, 0x00, 0x00, 0x00, 0x00};
uint8_t b[] = {0x00, 0x00, 0x00, 0x00, 0x00};
uint8_t error[] = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
 
uint8_t uart_buf[5];
/* USER CODE END PV */ 
 
while (1)
 {
 /* USER CODE END WHILE */
 
 /* USER CODE BEGIN 3 */
	 HAL_UART_Transmit(&huart4, a, 5, 100);
	 HAL_UART_Transmit(&huart4, b, 5, 100);
 
	 if(HAL_UART_Receive(&huart4, uart_buf, 5, 500) == HAL_OK)
	 {
		 HAL_UART_Transmit(&huart3, uart_buf, 5, 100);
	 } else
	 {
		 HAL_UART_Transmit(&huart3, error, 5, 100);
	 }
 }

The communication is between STM32 and an USB-TTL converter at 9600 baud.

The program send some bytes to the USB-TTL converter, the latter sends {0x05, 0x04, 0x03, 0x02, 0x01} and {0x0A, 0x09, 0x08, 0x07, 0x06} in repetition. The STM32 trys to receive the signal, if this is ok it sends to the ST-LINK uart the received pack, otherwise it sends to ST-LINK uart {0xFF, 0xFF, 0xFF, 0xFF, 0xFF}.

The STM32 receive the first pack of bytes, but than it returns always {0xFF, 0xFF, 0xFF, 0xFF, 0xFF} (ERROR). If you see the image, on the left there are the bytes sended by STM32, on the right the bytes sended through ST-LINK debug.

I can see the correct byte only when i reset the MCU, after the first read it return always error.

I don't know what is the problem, and i hope you can fix it.