My Xbox is wired and cannot connect while my son's xbox one is on the same network via wireless and has no problem.
We're both UpTo date on the Xbox and I just updated pihole to the most recent version today.
Turning off pihole fixes the problem instantly.
I'm at wit's end here!
Well, you edited your post to include this after I replied
Anyway, I also have no issue signing in on my xbox one, and I only have device.auth.xboxlive.com whitelisted! You could try clearing the cache on the xbox to see if that makes any difference...:
Clearing Cache
Unlike Xbox 360, the Xbox One does not have a menu option to clear cache on the console. Instead, follow these steps to clear cache on an Xbox One:
Turn off your Xbox One.
Once the indicator lights on the front of the Xbox One and the console's power brick are off, unplug the power brick from the back of your console.
Wait at least 30 seconds.
Plug the power brick back into your Xbox One and turn it on.