Comcast has released a statement saying that it is aware of an issue that is preventing some users from playing games online when connected over Wi-Fi. Microsoft and Comcast are working together to better understand the issue and they anticipate Microsoft will release a software update soon ("after the holidays") which should resolve it.
Apparently the issue affects only a very specific subset of users. If your Xbox One is connected to the internet via Wi-Fi, which also has an IPv6 address, you may be able to connect to the internet but not play certain games online.
Until the fix is released, Comcast says that connecting the Xbox One to the internet using an Ethernet cable should circumvent the issue entirely. If that option is not available to you, you may find further help on the official Xbox forums.