Apple TV 4K hardware--
Watching a basketball game when I was suddenly logged out. I have my DirecTV account username and password stored in my iPhone so I am able to get that into the DirecTV Apple TV app seamlessly. It fails! "Incorrect user ID or password. Try again. Care Code: 201".
I, too, have reinstalled, restarted, etc. And NO, I am not confusing my Apple TV, iCloud, or Apple ID credentials with my DirecTV account credentials. I understand the difference.
This is my main Apple TV in the house. And this issue seems to be only related to my Apple TV 4K. My 1080p Apple TVs do not seem to have any issues. I can watch the DirecTV app just fine on any other 1080p Apple TV in my house, through a web browser, iPad, iPhone, etc. My ATV4K is updated to 17.3 (21K646).
Unrelated, sometimes my ATV4K will get an error when I try to watch one of the 4K channels. I get this my device is not 4K capable but it is. I have found that if I turn off Dolby Vision and force my device to a certain 4K setting under video settings I can get it to work, some of the time.