I saw one Xfinity employee say to contact them to get whitelisted, however didn't see how to send a message. My website https://whatsyourelephant.org keeps getting this message:
This site can’t provide a secure connection
whatsyourelephant.org sent an invalid response.
I had no problem getting to your mentioned URL.
Perhpas something on your system - a quick Google search provided some genral information as to how to clear this error towards the end of the article.
Hope this helps some.
Does this pertain to Xfinity xFi Security? If so asking the question on the Xfinity xFi Board may get you more information.