Hi, Excuse my lack of German - I have used Google Translate
UnbeKannt is unkown in English So you are allowing public users, that is not logged in users, to download.
Do you want that to happen?
If you do not want public users to download then you need to set the Joomla! priveleges appropriately. The simplest way for a quick solution is to go to the jD panel and click on the Options button in the tool bar.
Firs select the Registered Group and set the Priveleges for Download to Allowed. Then Save - it is the Saving that sets the privileges and propagates them.
Next go to the Publi Group and set the the Download Privelegde to 'Inherited' and Save & Close. Do NOT set to Denied as that will stop everyone!