Can't login to github.com: sun.security.validator.ValidatorException: PKIX path building failed:

I have a local Git repo working nicely, and I also have a GitHub account (with SSH set up correctly); however, when I try to 'Share project on GitHub' and login to GitHub via the supplied dialog, I get error:

Can't login to github.com: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

after hitting 'Proceed anyway' to warning 'The security certificate of github.com is not trusted. Do you want to proceed anyway?'

I also get this same error when I go into Settings/VersionControl/GitHub and do a Test on login.