Smoothwall generates replacement certificates for websites on the fly and caches them for re-use. It might be that a site certificate has been replaced and the cached certificate that the Smoothwall created is causing this error.
- Clear the cache by going to Guardian menu under the HTTPS inspection submenu and click Settings.
- Click Clear and restart, next to the Cached certificates header.
- Close the web page and re-open to check if that has fixed the issue.