Question: Apple ID password quit working on one device
I just upgraded my Apple iPad Air 2 to iOS 11.3.1 from iOS 10.3.3
At the page I'm supposed to enter my Apple ID and password so I could set up iCloud, it showed the correct email address used for my Apple ID. I carefully entered my password, viewing each character as I typed it. I did this 3 times. Each time, it said User name or password is incorrect. But they ARE both correct.
I used the same Apple ID and password to successfully sign in on my PC.
iPad Air 2 Wi-Fi, iOS 11.3.1