Global administration - Panels, only shows code

Hi all,

after performing a clean install of xwiki (9.11.8 LTS) on Ubuntu 18.04.1 LTS all but the panel administration seems to work.
While logged in as the admin navigating to “Look & Feel” then “Panels” only shows a page full of HTML-code, see attached image.

How can I resolve this?

Regards,

Maarten

xwikipanels