I was experiencing the exact same behavior as @GrannyWilliams. In addition, the Comcast Business app on my phone was consistently failing to launch due to a "network error." Moreover, following the advice in the Reddit thread, the situation was resolved by briefly powering off my Arris SB6183 modem and the router behind it. The router's IPv4 and IPv6 addresses were identical before and after the power cycle. The only thing that changed was how the DNS name "oauth.xfinity.com" resolved. Before the reboot, it resolved to "oauth.g.xfinity.com" at 2001:558:fe16:109:96:114:156:145 . I could not successfully ping this address, either before the reboot or now. After the reboot, it now resolves to "oauth.g.xfinity.com" at 2001:558:fe21:38:162:150:57:167 . I can successfully ping this address. Hopefully this gives you some more data to work with.
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