I bought and activated a new modem (Arris SB6190) a few days ago. Ever since, I will occassionally get blocked from certain webpages due to because "connection is not private". Also I use a VPN for work and it sometimes fails to connect due to a "security" issue.
see the screen shot below for what I'm talking about. (EDIT: don't know why the image won't load, so go here: https://www.dropbox.com/s/49cp337hngcw1qg/bad%20certificate.png)
If I click Advanced -> proceed anyway, I get redirected to the Xfinity device activation page. It is as if Xfinity temporarily fails to recognize my modem and intercepts my internet traffic. After I re-activate my device, the original website will successfully load, but then the problem will reemerge a few hours later. I've probably gone through this process 10 times by now. I've also tried rebooting my modem several times by now. And I still get this problem several times a day. Also, if I do nothing and don't try reactivating the device, usually the problem will eventually go away on its own. Only to recur later in the day.
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.