PRTG Network Monitor comes with a default SSL certificate so you can securely use the web interface through HTTPS out-of-the-box. But because it is not an official certificate that matches the domain name or IP address of your PRTG installation a web browser will always show an alert ("the certificate is not correct") when it connects to this server (although the transmission is already secure). If you want this dialog to disappear you must create, order, and install a trusted certificate. Our latest knowledge base article explains how you can do this by getting a certificate from Comodo InstantSSL. Apart from official paid-for certificates they also offer free 90-day-certificates that work well for PRTG 7.