Hey @Enrico Rossini
Unfortunately, my account is restricted and therefore, could not step though Send notifications to Universal Windows Platform apps using Azure Notification Hubs | Microsoft Learn to confirm the issue. What I can suggest is try creating a new app in Developer Center and seeing that Package ID and Security Key is successful. If not, then you'll have to reach out to http://aka.ms/storesupport to submit an incident with Microsoft Partner Centre.
I have reached out to the team regarding this matter to see if there are any insights.
EDIT 9 Feb 2024 I got my account unrestricted and was successfully able to register my store app with Notification Hub. I suggest generating a new application secret in https://apps.dev.microsoft.com/ to see if that resolves your issue.