Strange volume issue

Im using a volumio primo hifi edition. With a external dac. I Can change the volume with the slider on the webinterface or via the app.

When using Spotify connect i Can change the volume via the buttons on my phone.

Volume mixer type is hardware.
Im running 3.179

A friend also owns a Volumio primo with the same settings active. And the same software version. Hé cannot change the volume via the app or webinterface of volumio

When using Spotify connect you Can see the volume slider moving. But Nothing happens.

Does anyone know Where this comes from?

Is the external dac the same and support volume mixer?

No. The external dac is not the same.Mine is the Exogal Comet dac. How Can i check if it supports volume mixer?