Comments from Google accounts on Twitter and Reddit say they are working on a fix.

Hey there, we’re aware of an emerging issue impacting Chromecast 2nd gen and Chromecast Audio devices and are working on a fix. Do not factory reset your device - we will keep you all updated when the fix rolls out. If you have already factory reset your device, we will provide instructions to set your device back up as soon as possible. Thank you for your patience.