Mixed Content issues occur when a website data is loaded on both HTTPS and HTTP connections. When an existing HTTP site is secured by installing SSL (Secure Socket Layers) certificate and switched over to HTTPS; all contents, videos, links should be properly loaded to HTTPS. Many times it may happen that a site has a few links or pages of HTTP attached to it, though the whole site is moved to HTTPS.
Upon issuance, all SSL certificates have a finite lifespan and are no longer recognized as valid upon expiration. Certificates may have varying periods of validity and are often set to expire anywhere between one and three years based on company policy and/or cost considerations.