Hi @Hatcat
Welcome to the Sonos Community!
I’ve removed your second post because it contained identifying info and only one item of use for troubleshooting. The second post also made it look like someone had replied, hence the delay in you getting an actual reply. Next time, should you want to edit your initial post, you’ll get 1 hour after posting to do so - click on the 3 dots to the right of the post to see the option.
As for the issue you report, the most common cause of seeing “the song is not encoded correctly” is actually a failed transmission to the device trying to play. The error makes the incoming, compressed feed look incorrect when the software is trying to decompress it into an actual digital audio stream. The usual cause of such a situation is electromagnetic interference. As your S1 system connects differently (via SonosNet instead of WiFi) than your S2 system, this would correlate with your differing experiences.
Please see our Reducing wireless interference help page for a guide on what to look for in reducing interference. As your S1 system probably has a single, ethernet-connected device, you should concentrate your efforts in it’s vicinity first.
I hope this helps.