Input/output error from HTTPS url: "unable to find valid certification path"

In case this helps someone else, I was receiving an error message when scraping an HTTPS site:

An input/output error occurred....sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target.

In the session's Advanced tab, changing the HTTP client from Apache to Ning solved the problem.