The new error indicates that you have the wrong parameters for your sound card. Make sure you have the right output device selected and please paste any other parameters of the player here as well. It looks like Spotify is using the built-in Pi sound card instead of the amp. Check to see that „sysdefault_hifiberry…“ is selected and no other player is running concurrently with the same card.
Also, as I’ve mentioned, the Squeezebox Server solution Spotty is a great multiroom-capable alternative that also supports Spotify Connect.
Here’s a tutorial on setting it up