Assets: Download as ZIP results in "success: true"

Pimcore shows a strange behaviour in Digital Asset Management: When I have the list-view and click on multiple single entries checkboxes and afterwards on “Download as ZIP”, I get forwarded to for example and the browser shows the JSON statement {“success”:true}.

The respective user/role has only rights to list and view assets - is it a right-based problem? And if so, what needs to be changed?