Chrome users on Mac OS X may experience an error when accessing https://gandi.net or other websites. This error will effectively prevent you from using the website.

To fix this error, please reproduce the steps described below and in this Youtube video:

1. Open “Keychain Access” on your Mac
2. In the “login” section, delete two certificates : “UTN DATACorp SGC” and “AddTrust External CA Root”
3. Quit the “Keychain Access” and “Google Chrome” applications
4. Reopen Google Chrome and visit https://gandi.net

(Thanks to David)

AVAST on Mac does a man-in-the-middle attack on HTTPS connections, so you may well see your Chrome protest and it has nothing to do with actual certificates but with the fact that AVAST presents unverifiable fake certificates to Chrome:

http://www.thesafemac.com/avasts-man-in-the-middle/

http://www.ghacks.net/2014/10/31/avasts-https-scanning-interferes-with-firefox-and-other-programs/