Trying to enrol iOS/iPadOS device to Apple's profile manager ALWAYS fails with wrong login error

We setup a Mac mini running macOS Catalina and macOS Server. We connected the system to our Active Directory and configured the profile manager (it uses a certificate from Let's Encrypt to secure its services). We can access the server homepage fine and log in using either local accounts and domain accounts.

Now when trying to prepare iPhones/iPads using Apple Configurator 2 we always get stuck at the step when the device asks for the credentials to login into the MDM. Whatever credentials we enter (local admin/user, domain user) it always complains about them not being correct. Again opening Safari and the profile manger homepage we can login just fine and register our devices (however, they are not supervised using this method).

I attached a screenshot of the error message (in German though). It reads: "Configurator failed to load and apply the cloud configuration because the MDM credentials are wrong. Please enter your credentials for <myprofilemanager>."

Does anyone know how to solve this or explain what the problem is? Any pointers are highly appreciated.