Cairo-Dock recognizes App when starting after App, but not other way round

[nuevoruido]

I installed Cairo-Dock and Spotify on Xenial. When I start Spotify from Cairo-Dock it is neither shown as active (if I click the icon again, another Spotify instance starts) nor an extra app-indicator is shown. I can only go back to Spotify using Alt+Tab. When I start Spotify before Cairo-Dock, Cairo-Dock does detect it to be running and shows it as active!

I started Cairo-Dock both with and without OpenGL. No idea what else to try ...
Anyone? Thank you!