Won't login to treasury direct

Over the past two years I have used treasurydirect.gov to purchase savings bonds. Last week, for the first time, I couldn't log into my account. This drop down message appeared: Safari can’t open the page “ https://www.treasurydirect.gov/RS/BPDLogin?application=rs” because it couldn’t establish a secure connection to the server “www.treasurydirect.gov”. I'm running safari 10.3.9

I tried on my son's computer and received an unknown error message 10054. He is running 10.4
Thanks for any help you can give. John