On 10:48:38 UTC, May 30th, 2020, the AddTrust External CA Root certificate expired. If you have received alerts for Pingdom checks about the certificate of your site or server having expired it is most likely due to that specific certificate being installed on your site. Many websites and services have been impacted by this.
This certificate does not affect modern browsers so checking the certificates in your browser will unfortunately not highlight it, and Pingdom's Root Cause Analysis does at the moment not point directly to the offending certificate. Pingdom checks do, however, monitor all certificates installed on the URL/IP set up to be monitored and alerts if any of these have issues.
This article from Andrew at sslmate.com explains what has happened and how to fix it.
As soon as this certificate is removed from your site, your Pingdom checks with Certificate monitoring enabled should start to show as UP again, provided there are no other errors of course.
During Monday, June 1st, and Tuesday, June 2nd an update was made to Pingdom probe servers across the world. This update included an update to properly handle cross-signed certificates that was previously unavailable. If a check using the default region of North America and Europe was showing UP from one region but DOWN from the other this happened during the update time. Provided your site has either removed the offending certificate, or has a cross-signed path that works, there should no longer be any certificate expiration warnings for this specific certificate.