I had this briefly with duckduckhack.com yesterday but it works now and duckduckgo.com is fine.
You could try re-installing the relevant certificates which are issued by DigiCert and available here:
Click on the "Secure Server CA" and "Global Root CA" links and download the PEM files. If they're not already installed you'll get a dialog box prompting you to install. For trust, the three checkboxes should be all checked for the "Global Root CA" certificate and all unchecked for the "Secure Server CA" certificate.
Alternatively go to Preferences -> Advanced -> Certificates -> View Certificates -> find the DigiCert section. You could try editing or deleting/importing the "Global Root CA" and "Secure Server CA" certificates as above.