(Advanced) Custom Field Widget not working

Hi,
On a website I like to use the Custom Field Widget or the Advanced Custom Field widget. I’ve installed the CF widget, but I can get a widget, but there is no content in the widget, it’s blanco. No fields to set a key or anything. So I installed the advanced version.. same story.
Do this plugin aks something extra from the server? I asume they have the same base since they show the same problem.

Hope someone recognise the problem and know how to solve it

Thanks

Karin