Can't activate Showtime Anytime on one of my Apple TV devices

I have 2 Apple TV 4k boxes. Both have been working fine and I've watched Showtime Anytime many times on both. In the past couple of days though, on one of the devices, it appears that the app as been unlinked despite the fact that in "Settings," SA is set to always use my TV provider (ATT TV).

When I go into the app, I'm prompted to activate and given an activation code. I then go to my laptop and walk through the activation process: entering the code and signing into my ATT TV account. However, I then get a "Login Failure" message which references something about a single-sign in problem. I uninstalled/reinstalled the app multiple times, restarted my Apple TV device multiple times and verified that on my other Apple 4k device - as well as on my iPad and iPhone, Showtime Anytime is properly linked and working. The issue is just happening to this one single Apple TV device.

Any ideas what the problem could be?