Hi Volker,
Thanks for contacting Syncfusion support.
As per our records, you have downloaded and installed Syncfusion licensed Blazor installer. So there is no need to generate and register the license key in your application. You can remove the below statement from your application and can run.

The licensing is applicable for all the evaluators and the licensed customers who use NuGet packages from nuget.org. If you are using the Trial installer or NuGet packages from nuget.org, then you should generate and register the corresponding version license key in your application. Refer the following documentation link to know more about licensing.
https://help.syncfusion.com/common/essential-studio/licensing/license-key Let us know if you need any further assistance on this.
Regards,
Christopher H