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Chromecast (2nd gen) and Audio cannot Cast in 'Untrusted' outage
(9to5google.com)
This is a most excellent place for technology news and articles.
It's possible to encrypt traffic without a cert that expires.
I don't know much about how Chromecast works but my bet is that it's like a hacked together lighthttp or nginx server, which is why it needs a SSL cert renewal.
If I remember correctly I have seen an nginx page in combination with some chromecast device.