We ran into this same issue for the first time yesterday too. Xfi Advanced Security started blocking our websockets endpoint, websocket.carsandbids.com/carsandbids. Our logs just showed a couple users failing to make the connection. We never would have figured it out except that one of our employees was getting blocked and was able to help us debug.
We submitted the url to Xfinity ⋅ Service Policy Assurance to get whitelisted, but haven’t heard back yet, the form said estimated three business days.
matt at carsandbids dot com