It seems there were some changes at the keychain in macOS Sierra, after upgrading git seems not to be able to find the client certificate required to connect to our server.

fatal: unable to access 'https://xxx:8443/git/proj.git/': SSL: Can't find the certificate "John Doe" and its private key in the Keychain.