iPhone asks for user and password when applying registration profile (remote management)

We are using the apple profile manager to manage more than 100 iPhones and iPads in our organisation.

Since 3 or more weeks the registration profile (remote management) could not be applied to the iPhones, because user and password are required. None of our credentials works:

The iPhones are brand new and never had been used before.

Some months before the blueprints (including registration profile for remote management) could be applied with the apple configurator 2 without asking for user and password.

What's wrong ?

Greetings