Ipod dock volume controls buggy while/after using Spotify Mobile

I've bought a Logitech s315i Ipod/Iphone dock recently. The problem is, that the moment I start Spotify mobile app on my Ipod Touch (1-st Gen) the loudness button (+) on dock stops working. When I press on it, the Iphone volume control display appears and volume jumps down instead of up. Normally I would blame my dock for that, the thing is that as long I don't start the spotify app (e.g. playing music on Ipod in a standard app) volume controls on the dock work correctly. So somehow Spotify app is causing this behaviour :/
sorry if it sounds a bit cryptic :)