Maybe, this whole issue is trending on a 6 year old reddit thread where gen 2 randomly started failing world wide today. Hope we can get a response from google on how to fix this or if they are completely gonna axe the gen 2s.
I just spent 2 hours resetting and debugging my 2nd gen HDMI dongle. Blessed with having a UniFi network which can be a bit temperamental with Chromecast. For me the song and dance started with "outdated device firmware".