Veeam ONE uses TLS to ensure secure data communication between Veeam ONE Reporter website and a web browser.
When you install Veeam ONE, you can generate or choose a self-signed certificate. In this case, when you try to access Veeam ONE Reporter from a remote machine, the browser will display a warning notifying that the connection is untrusted (although it is secured with TLS).
To eliminate the warning, import the self-signed certificate to client machines (the machines from which you plan to access the Veeam ONE Reporter website).
For details on importing TLS certificates, see this Microsoft KB article.