www-jstor-org.libproxy.uncg.edu:443 uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.

Your library's proxy server is misconfigured, and their IT people will need to fix it. You can send them to https://www.zotero.org/support/kb/incomplete_cert_chain for an explanation, and they can view the SSL Labs test results that show that it's configured incorrectly ("Chain issues: Incomplete").