Resolved! STM32H735 UART2 HAL_UART_Receive_IT and HAL_UART_Receive puts a first byte in the buffer that is not there.
I am a bit frustrated with the USART2, which is connected to the UART9 TX9-RX2 and TX2-RX9. So with sending from TX9 to RX2 everything is fine but the other way around there is a first byte = 254d inside the receive buffer. huart2 and huart9 are init...