We generally recommend Spotty for all multiroom purposes due to its modular structure in the Squeezebox system.
In our Spotify Connect player, the error you name is not really an error and should not cause the issue of being unable to stop. This is most probably due to multiple players trying to occupy the same sound card and running into occupied ports. You can check this by stopping all players in the Audioplayer menu and restarting the Spotify Connect player. However, your best bet with multiroom is Spotty, as I mentioned.