HTTPS Scanning works by inserting a certificate in your browser to allow the secure traffic to be intercepted and scanned. Some sites might not work well with this method, but its designed in a way that should be transparent to the site you are visiting.
Site <>Avast Engine <>Browser The connection between the site and the engine is encrypted and so too is the connection between the browser and the engine.
To test if this is the cause of your issue with a site, disable the web shield and then close and relaunch the browser. You should see the site's SSL Certificate instead of the one from the web shield