Answer: Although the warning looks alarming, there is no reason to think that your server is insecure.
Both cPanel/WHM and Plesk use secure connections on their control panels; the security of those connections is provided by SSL certificates. Out of the box, both Plesk and cPanel use a type of certificate known as a self-signed certificate. Self-signed certificates work exactly like a certificate purchased through an SSL Certificate Authority, except that they are NOT signed by a Certificate Authority. Instead they are signed by your server; hence the term “self-signed”. That is the only difference. Apart from that, the encrypted connection using a self-signed certificate is as secure as any other SSL connection.
So why do most browsers throw that scary-looking error? The answer lies in the role that certificate authorities play. Certificate authorities perform various checks to see that whoever is purchasing the certificate is who they say they are. Browsers assume that a site that uses an SSL certificate that has been vetted by a certificate authority is safer than a site that does not.
So please accept the connection and add the site to your list of accepted secure logins and the warning should not appear again.