If a WebTitan certificate has not been installed and a user visits an HTTPS-enabled website that is blocked by a policy, their browser will return a certificate error page instead of your WebTitan block page. To avoid this, download and import the WebTitan SSL Certificate.
Go to Settings > Certificates.
Select the Download button under SSL Certificate to automatically download ca.der.
Once downloaded, the certificate needs to be deployed to all computers that are being filtered by WebTitan. This can be done using Group Policy (GPO) or any other similar software.