I know what causes it. Xfinity tries to ID people above and beyond the credentials, by looking at the network connection, IP address, and probably browser fingerprints and who knows what else. It doesn't like people coming from other networks than its own. So if you're on mobile, or let's say, VPN, it can give you problems.
To test the thing, I had my home network on VPN -- tried to log in, got the loop. Killed the VPN, and was able to log in just fine. It doesn't really matter what VPN it is (commercial or corporate).
Customer service is clueless about this stuff, don't even bother unless you want to spend the next 2-3 hours with them trying to help but taking you nowhere. I've done it.
What's the point of having passwords if they force logins on their raw network? Very disappointing.
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