This warning appears because the encryption certificate of the locally installed digitalSTROM server is generated by the server itself and is not issued by an official certification authority.
A certificate is always issued for only one address. Since the address of the dSS can change at will (https://dss.local, or the name you have given it), the dSS issues itself a new certificate to match if its address changes. This is a common procedure for devices installed at home, such as routers, webcams, etc., if they want to offer an encrypted connection. Many devices like to do without encrypted and secure communication and in this way "spare" the user with the browser warning messages. Since digitalSTROM is particularly secure, however, only encrypted communication with our servers is permitted. The connection is therefore not insecure, quite the opposite. The browser only reports because it cannot check the issuer of the security certificate itself. If you now read the warning displayed with this knowledge again, you will notice that the browser only issues a warning because it is not completely sure.
We recommend that you use Mozilla Firefox as your browser. There you can accept the certificate the first time you connect to a dSS and then no further warnings about this server will be displayed.