Questions about Vogels 6VO SAVA1014

Does anyone know the exact function of the red LED. The unit does not work for me and the LED behaves strangely. When it was working correctly, I expected it to light up according to the reception signal from the IR-FB, but it does not. When plugging the mini-USB into the mains adapter without the ext. LEDs, the red LED lights up brightly. When the external parts are plugged in, it goes out more and more?