No sound via at I2S using Spotify plugin

I’m using 2.296, rpi3, Generic I2S DAC (using my own DAC-board). Sound from other media work fine via I2S as well as sound via spotify plugin (20170730) to the rpi3 audio jack. But no sound to I2S using Spotify plugin. I have tried several combinations of settings as well as a fresh install, but still no sound. What should I try?