Okay it seems that I have found the error...

The error is in the call ‘$app->withFacades();’ within the app.php.

This causes the classes within the lumen ‘Application.php’ to be declared twice. If you call ‘withFacades’ with the parameter ‘false’, the classes are not declared again and the whole thing works again.

I then called withFacades again with the parameter ‘true’ and deleted the standard alias ‘‘Illuminate\Support\Facades\Event’ => ‘Event’,‘ and “”Illuminate\Support\Facades\URL’ => ‘URL’,’ within the Application.php. I had to delete the alias “URL” as well, as this caused the same problem.

Unfortunately, I don't currently know whether this is a bug in the Lumenframework or whether there is an error somewhere in my own code. At least it is currently working again :)