Pdf export of a task page is empty

To reproduce:

• install task manager application (I tested on XWiki 6.4.5 but might reproduce on other versions too)
• create a task, fill in some information about it (description, etc) -> see attached image
• go to the task page, export as pdf

Expected result:

• the information about the task should be present in the exported pdf file

Actual result:

• the resulted pdf file is empty -> See attachment