Invalid Root Certificate (Comodo RSA)
The COMODO RSA Certification Authority certificate (Serial Number 4C AA F9 CA DB 63 6F E0 1F F7 4E D8 5B 03 86 9D) is not a default trusted root in El Capitan. You can find the list of the trusted root certificates in El Capitan at https://support.apple.com/en-us/HT205204

That root became trusted in Sierra and later. You can find that list of the https://support.apple.com/en-us/HT205204. 

Either upgrade your macOS, or install the updated COMODO SHA-2 roots and intermediates from  https://support.comodo.com/index.php?/Knowledgebase/List/Index/71 and install them as X509 roots in Keychain.

Start with https://support.comodo.com/index.php?/Knowledgebase/Article/View/969/108/root-comodo-rsa-certification-authority-sha-2

Download the .crt file, click on it, and follow the instructions at https://www.askdavetaylor.com/how_to_install_trusted_root_certificate_mac/

Alternatively, you can use Firefox, which maintains a certificate store independent of the one in your outdated Keychain.