HBO Max app stuck in an endless "Can't Verify" loop

The HBO Max app on my phone is stuck in an endless "Can't Verify Your Subscription" loop. I launch the app, it pops up a dialog that it can't verify my subscription, I dismiss the dialog, then it immediately pops up another dialog, etc. I can never get to a point where I can re-enter credentials.

I have deleted and reinstalled the app a couple of times. I've even power-cycled the phone in between installs.

I have my account directly with HBO Max. This is not an App Store subscription.

The app works fine on my iPad and Apple TV, and I can log in and watch via Safari on my MacBook with no problem. It's just the phone. I'm running iOS 14.4.2.

Does anyone have some ideas?