Cannot find expired certificate in Keychain Access
The procedure to locate an expired certificate in a specific keychain is covered well in this answer:


How do I fix the SSL warning I get when visiting github.com?


Be sure that you look on the left of the Keychain Access window - you should have a login keychain, maybe an iCloud keychain, a system Keychain and System Roots.

If you don't find the expired certificate in the first keychain, repeat the process after selecting each other keychain. Also pay attention in case you are not selecting All Items or Certificates in the filter under the list of Keychains.