Hi @Der_Alte_Fritz 


The error which you are getting while setting up Teams is the error that generally comes when some other Office 365 Admin has already created the Microsoft Teams Services for the user accounts of the Tenant. Do you have multiple Office 365 Global Admin in your company? If yes then please follow the below steps and see if you can log-in?


1. Download the Microsoft Teams Application on the desktop and use your normal Office 365 Login and see if you are able to login to Microsoft Teams using the same? or

2. Try logging into the Microsoft Teams Web Applications teams.microsoft.com and see if you are able to login there?


If both of the above steps fails check if you have been assigned with the Microsoft Teams Licenses in your organization.


With Regards,

Satish U