When accessing an encrypted website (HTTPS), a "certificate not trusted" message in the browser usually occurs because the certificate presented by the website is not issued by a trusted Certificate Authority (CA). A common scenario for this is when a self-signed certificate is installed on the server. Self-signed certificates are not signed by a CA that is recognized by the browser, so the browser cannot verify the authenticity of the certificate, leading to the "certificate not trusted" message.
References:
* "If the SSL certificate is self-signed, the browser will not trust the certificate because it has not been issued by a trusted CA.".