I have the same problem. Everything is up to date date. Everything is on the same network. The only way I can fix it is to clear storage in the Spotify app in Android settings. When I do this I can cast to my Chromecast Audio for a few hours. Then when I come back it has stopped working completely. My other music apps don't have this problem so it must be your app. Please fix it quickly!
Deleting the cache and program files resolve the problem in the first relogin to the app. But once you kill the Spotify process on the task manager and re-enter again, the problem comes back again... Shameful
Same problem, chromecast ultra works flawlessly with every other app except spotify, that speaks for itself really. I can cast every once and a while but once it locks itself out you are stuffed. Spotify is actually my most hated app in terms of bugs currently. This should be a priority 1 bug IMHO.
I've been having on going issues with this as well. Using latest versions of everything on android, I seem able to play for a bit then it gets lost and can't be found. Desktop version of spotify most of the time can't even find my chromecast devices. Google home shows them and I start spotify that way but no luck getting things to sink back up. I have a chromecast audio and newer chromecast with HDMI. If I send to both in home group it seems to work better. I also tried Google Home to cast audio to devices, then play songs from spotify on phone, that worked for 1 minute.
When I select the group I can hear the chromecast audio make a noise, and the hdmi displays that it has connected to spotify, but it doesn't play music.
I finally sent spotify music directly to my Onkyo amp, then I was able to select the google home group and have it work.