xfce4 panel plugin does not show anything

I installed workrave-xfce, and tried to add the workrave item to the panel. However, it does not show anything. On adding a single workrave item, the icon is grayed out so you cannot add any more, and the item in the panel items list indicates "Workrave (external)" but nothing is displayed. If I remove the item from the panel, the executable xfce4-workrave-plugin is still running as a process.
Running on Ubuntu 14.04 i386, with workrave 1.10.6.0