Volume slider not working with hifiberry digi clone


i have a RPi2 with a DollaTek HiFi Digi+ digital I2S sound card (https://www.amazon.fr/gp/product/B07DK5C4TY), linked by the RCA connector to my digital amplifier. This is an HiFiBerry Digi clone board.

This setup is working perfectly, except that the volume slider in the web interface is not producing any action on the audio output volume.

Anyone can give me an advice on how to configure my pimusicbox distro for making this slider working?