How do I fix Safari's 'This connection is not private' error for medical and financial sites on my iMac?

Recently not able to sign into Medical sites (ex Anthem & Bristol Hospital Patient Portal) and Financial sites (ex. Experian etc) I am able to sign in with username, password and 6 digit code sent via text but the screen goes black with a message: "This connection is not private This website may be impersonating "secure-gateway...." to steal your personal or financial information. You should go back to previous page."

I contacted Anthem support and was told that they are having problems with Safari. My bank told me the same thing and to use Chrome instead. I have Norton360 and Lifelock installed - the same error message appears when the VPN is both on or off. This started after the last system update. What can I do if anything on my Imac to correct this issue? Will appreciate any and all feedback!

[Re-Titled by Moderator]