I can use Spotify Connect from my phone, but when I use Spotify Windows App (v 1.220.1216.0) to connect to my local LAN Volumio player, it shows green for about 2 seconds, then kicks back to black and plays via local computer.
Welcome to Volumio for Raspberry Pi (5.10.92-v7+ armv7l)
volumio@volumio-allo-digione:~$ cat /sys/firmware/devicetree/base/model
Raspberry Pi 3 Model B Plus Rev 1.3volumio@volumio-allo-digione:~$
The Spotify Plugin on Volumio web GUI shows 3.0.9.
Here are couple of screen grabs of the spot in the Windows App

The little icon just won’t turn green and connect!