You will see this if you're accessing your Squarespace site using SSL (Strong Security) under a custom domain (https://), or accessing your billing information from a custom domain. The warning displays because the certificate you're using is provided for Squarespace.com, rather than one purchased for your mapped domain specifically. While your traffic is still encrypted to our system, this warning is informing you that you may be contacting a host other than the one you intend. Unless you run a very high-risk domain, this is not happening to you. You can avoid this warning entirely by turning off the SSL option or logging in using your squarespace.com URL.
Note: Squarespace 5 is our legacy platform from 2012. Although we still support Squarespace 5, fixing bugs or issues is a lower priority for us while we focus on our current platform, Squarespace 7.