"Request failed with status code 400" in Partner Center when visiting "Xbox development...

So I have finally built my first D3D UWP application and wanted to test it on my Xbox. For that, I instructions on Xbox Developer Mode activation - UWP applications | Microsoft Learn, downloaded the dev app, went to https://partner.microsoft.com/en-us/xboxconfig/devices/index and found out that the page immediately showed a red notification bar "Request failed with status code 400". If I try to add the device via the plus button on the right, it also shows "Request failed with status code 400". I know that the Microsoft account I am using is a dev account since the old times of Windows Phone and I have several apps listed in the dashboard - I also can create new apps.

The instructions from Xbox One Developer Mode Not Activating (Website Does Not Display - Microsoft Community also do not help. I do not need help finding the plus button, I need someone fix the tons of JavaScript errors my browser shows when I go to the "Xbox development consoles" page.

:)