This is what happened to me, and it may happen to you too: 

The comment I got from the tech guys, and it still hurts: "Good catch!" You have printers, but no materials attached, system is stuck". And they were correct:
Back office:
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As soon as I added a material to the Ultimaker the widget worked again, obviously showing only that material. Long story short: For the widget to work correctly, you need at least one printer with one material attached.