xbox live login broken

How do I get any Microsoft service on my sons iPad to actually provide me a login prompt?

Started with Minecraft

Did not log in with original credentials anymore. But also will not show me a login or password text box to try entering different credentials either. Have tried force-quitting the app (switcher with a swipe up), Power cycling the device after a forced quit of the app, and even deleting, powercycle, and reinstall of the app. How is one to fix a broken login/password cache if the app never shows you a login prompt, and somehow preserved the credentials even after the app is deleted and everyone power cycled???

Moved on to Xbox App

Found guidance that included instructions for logging out of the xbox app on the device as well, then logging back into the xbox app on the device before trying Minecraft. Ok, so I installed that app, but when I press the "log in" button it just goes grey for about 30 seconds. Nothing else happens. I have raspberry Pi running on my home network, and I can filter by his IP address. As best I can tell his device is making no outbound DNS requests, suggesting it isn't even trying to contact MS servers (unless it has them hard coded as a string of numbers, which I find hard to believe).

Checking Xbox Live webstie

logged into the xbox live website to see his login activity. Thinking I might see the failed login attempts and get some guidance from there. Maybe MS was blocking them because it thought they were fraudulent or something, but nope. Despite having an xbox associated with the account, no login attempt are listed EVER. Not just for today (which I've tried at least 2 dozen times now), but never as far as I can tell from the UI.

So I reiterate, what can I possibly do to get the iPad to forget his apparently corrupted login-credentials so that it'll acutally present me with a login-prompt? I'm this close to giving up and telling him he does not get to play Minecraft anymore ever again because I'm out of next-steps.

:)