I can Confirm a fix that worked on both a 10.9.5 & 10.11.6 Mac OS users. Simply set the DST Root CA X3 to "Always Trust" on several Mac's I manage in an office and home's this fix work for 4 websites that previously had issues with this CERT ERR.
Directions for fix:
- Open ~/Applications/Utilities/Keychain Access.app
- From View menu select "Show Expired Certificates"
- On the Left Sidebar pick System Root
- In search bar top-right type DST
- Double-click "DST Root CA X3"
- In pop-up, turn down "Trust" arrow and set "When using this certificate" to "Always Trust"
- Close the pop-up and put in an Administrator user/password info.
- Close all open Browsers & Keychain you should be good to go after that.
