Google seem to be aware of the issue, and are working on fixes, both for
people who haven't factory reset Chromecast and for those who
have.https://www.googlenestcommunity.com/t5/Streaming/Regarding-an-issue-with-Chromecast-2nd-gen-and-Chromecast-Aud...
Same problem as in this one:
https://www.googlenestcommunity.com/t5/Streaming/We-couldn-t-authenticate/m-p/437872but
since the comments are disabled and the issue hasn't been resolved (the
current accepted answer does not cover my, and I'm sure many ...