Resolved -
The monitoring tool has confirmed they have resolved the issue.
Jun 22, 12:32 PDT
Monitoring -
We're continuing to monitor this but have no indication that browsers are rejecting our valid TLS certificates -- the issue appears to be in how our monitoring tool interprets the validity window. We will keep this open until they report back and we see the intermittent errors no longer showing up.
Jun 22, 08:25 PDT
Investigating -
Our CDN provider is investigating an issue with occasionally serving a TLS certificate that is either expired or only valid starting a small distance in the future. Our TLS certificates are provided by Lets Encrypt.
Jun 21, 10:40 PDT