HifiBerry DAC plus no sound

I have an older HifiBerry with DAC plus running Volumio as AirPlay device. For a long time it worked fine. Recently, I wanted to use it again and found that it was not visible as Airplay device on my iPhone. I tried to upgrade the software which worked but did not help. So I did a factory reset and now I see airplay but no sound comes out. I managed to get choppy sound out of the device by choosing a different DAC. With HifiBerry DAC Plus, nothing. What could be wrong?