I just purchased a new Arris SB8200 cable modem from Best Buy. After activating the modem with Comcast/Xfinity everything worked fine for a day. The next morning, some, not all, web sites were reporting a privacy error and would reroute to register.be.xfinity.com. If I went to register.be.xfinity.com I was able to register the modem again and the problems cleared up. After about a week of doing this every morning, the modem registration process stopped working, showing that I was in a "WALLED GARDEN" state, and asked me to call Xfinity support. At that time I could get to about 75% of sites on the Internet without seeing the privacy error and being routed to the registration page.
I called Xfinity support and they are telling me it is my WiFi access point, not the modem, and not Xfinity. I'm not convinced. I'm sure it will happen again. When it does I'm going to connect directly to the cable modem to see if the error is occurring without the web access point in-between. I've seen others with similar issues on this forum and other forums on the Internet. I do believe that Comcast/Xfinity is having issues with their network and my modem configuration.
Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete.
I've had Comcast re-provision the modem several times. I was called by tech support yesterday and they told me my modem is not the problem. I'm being told now the random issues with https certificates and re-directs to register.be.xfinity.com are an issue with my new Netgear Orbi wifi router. There is nothing wrong with my wireless access point which has and is still working perfectly well. The issue I've been experiencing is related to Comcast and how they provisioned the modem or possibly how they are managing their internal network and infrastructure. I changed my DNS server configuration on my Netgear Orbi to use Google's - 220.127.116.11. Since doing that I haven't had any issues.