I am having this same problem on 2 different laptops. Doesn't matter what browser I use. It happens a lot when do a search and then click on the hyperlink that should take me to the page I want. Instead of going to the page, it takes me to the activation page.
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 have this same issue and the tech on the phone could not fix it despite reprovisioning the cable modem. This started happening the day I received a new Arris SB8200 cable modem. Based off of the last thread, in which several other people had the same issue, it sounds like this is something systemic to getting a new cable modem set up on xfinity.