This was a large Certificate Authorities root cert that expired over the weekend. It had a ripple effect that impacted lots of things. The underlying issued certificates were all fine, but when the root goes bad, everything goes bad.
They likely reissued the cert with a more current one, and it's been automagically rippling through the interwebs and cleaning stuff up.
I have now gone around this with the 2015 machine, three red flags to go past.
The only update available for me was an update on iTunes, something about third party "air play" speakers fix....