Same issue as other posters but for me it didnt fix itself 'after a few days'. All worked perfectly untill some days ago:

Running on LG-G5 the latest version of Spotify premium; Spotify app sees my chromecast audio and video devices but cannot connect to them anymore. Spotify App is able to connect to my PC via same WIFI. Spotify even tells me it found a cast device and asks me to connect to it...Firmware of cast devices all up to date. Note that other music and video apps connect perfectly to the chromecast audio (and video) devicesso it's Spotify App issue... When trying to connect Spotify App to chromecast(video) the TV switches on so there is at least some conenctivity but seems that connection with cast device cannot be completed...

loggin in/out, Clearing files/cache, reinstalling Spotify app (incl. restarting devices), rebooting/factory resetting chromecast, etc etc. all tried this but nothing helps. 

Anyone got another solution I can try, since I like to listen to some premium paid music again 🙂 ....thanks!