A fools errand here folks. It's a weakness in spotify's back-end. This is an error where spotify just can't handle the load and fails. It's all about their network and back-end and probably has nothing to do with their code, since it works fine and then randomly you'll get this error.
I don't think this is the case. Their desktop and iPhone apps presumably use the same backend and don't appear to have this issue. Sadly iPhones aren't affected so at this point I think this is very unlikely to be resolved....
....aaaaaaand the problem is gone, since I've switched to a new device (from a Sony xperia z5 compact to a Nokia 7 plus). Setup is the same (Android, offline songs stored on the same sd card that I used in my old phone), but suddenly Spotify is always online (and can always find my chromecasts). Apparently, the problem has *something* (I know, but I can't make it less vague than this) to do with your device and OS (and not *only* with Spotify).
Nope. The first logical thing to do that everyone already knows is to update it. Doesn't**bleep**. Spotify is too slow, and has too many errors. And now they made it worse by not having the choice to un-install it. What a bunch of mother**bleep**ers.