Yesterday, whenever I tried to load any page from the Internet, I was confronted with browser warnings that it received a certificate that it did no trust.
This certificate was issued from register.be.xfinity.com. No matter where I went with SSL, I encountered this. I checked my firewall settings and found that as a DHCP client it received new DNS servers:
18.104.22.168 -> susilanding-chic-vip.sys.comcast.net
22.214.171.124 -> susilanding-denv-vip.sys.comcast.net
I've always previously received
126.96.36.199 -> cdns01.comcast.net
188.8.131.52 -> cdns02.comcast.net
I changed back to the 75s locally on my firewall and resolved my problem
I had this exact same problem and I am so glad I found your message after working with support for 8+ hours and scavaging the interwebs. Thank You! I have no idea how that DNS entry was added.
Just to add context to this issue for future people who search for this issue, the SSL popup I was getting on my computers and mobile devices was:
Cannot Verify Server Identity
The identity of "outlook.office365.com" cannot be verified
The details of the SSL cert were as OP stated: register.be.xfinity.com
Removing the 184.108.40.206 DNS entry solved the issue.