For a few days it's the same behavior with my wifes Spotify account. Two Google Home Mini and a few Google Cast devices (AndroidTV, ChromeCast, Receiver and Google cast enabled media player) won't work. "Connecting..." is showing in the app and the Home devices make the connection sound, but streams won't start. Other apps (e.g. TuneIn or our local media server) are working instantly.
My device mostly works, but sometimes shows the same behavior.
My wife has a Moto G6 running Android 9, I have a OnePlus 6 running Android 10.
WiFi access point is an Ubiquiti AP AC Lite with 2.4 and 5GHz WiFi enabled.
When I logged in on my Pixel 3 under a different user on my family account, I was able to cast to devices on my home wifi network. I've opened a trouble ticket with Spotify for this issue which seems to be associated with my user account.
UPDATE: Seems like this is the issue I've been experiencing: