Quick and dirty, recently upgraded to the 5.0.6 (now 5.0.7) Unifi controller at home and enabled bandsteering on the Ubiquiti UAP-Pros I have. Immediately dumped the first-generation Chromecast. Disable band steering and the chromecast plays just fine. I have two Chromechast Audios running, which I believe to be similar hardware to the second-gen Chromecast, and they were fine with band steering enabled.

As far as I know the first-gen Chromecast doesn’t do 5GHz so it may have been erroneously being redirected to a band it can’t access. I haven’t seen that behaviour with any other 2.4GHz only clients I have.