I found a way just now to be able to stream ur spotify on the chromecast. I was having the same issue. So I try to connect and it shows spotify and then decides to call it quits. I went into the google hone app and ubder the chronecast name it would say "play music" try that
I had the same problem, in my case I got the problem for downloaded tracks. When I switched to a not downloaded track it worked, and then I could switch back to my downloaded track and then it worked too! Using Moto G4.
I had this issue...it was driving me nuts and even more so when I realised it depended on what album I was playing. It connected on one then when I tried to switch to another I'd get the women's voice saying the music has stopped because Spotify was playing on another device (it wasn't) and it disconnected from the Chromecast. Solved the issue by removing the albums that wouldn't play from my library then finding them again using the search function. They then connected without issue. None of the albums I attempted to play were downloaded, just saved to my library. I'm glad I worked it out but not very impressive update from Spotify. Was almost cancelling my subscription of many years.
3 devices won't cast to chromecast or chromecast audio
Samsung Galaxy Tab A: Android 5.1.1
Nexus 5: Android 6.0.1
Moto G: Android 4.4.4
Spotify on my daughter's Pixel phone will also not cast. Spotify on my wife's iphone SE has no problem casting.
All other apps on my devices cast with no problems. Only Spotify.
This problem has been happening for months. I used to be able to get it to work by clearing the Spotify App Data and then re-logging in to Spotify. Casting would work for one listening session. The next attempt wouldn't work.
I have tried all the suggestions. I'm paying for the Premium Family plan. But, it seems idiotic to keep on doing it, with such a fundamental problem going unresolved for so long. Spotify is a great service, otherwise.
Yesterday, I tried spotify on my phone at my sister's house. It connected to her chromecast audio right away! Got back home and it wouldn't connect here.
I also noticed that I can simultaneously log into spotify on all 3 devices and the web app and play 4 separate streams at the same time. When things were working properly, this wasn't allowed. Maybe that's a clue?