Get the same error message after upgrade to 2016.1.
Happens if i want to choose a Support Group for a Change request.
Update: My reason was the Name Attribute because there was an ü instead of ue used.
I created a new relation between Object "User" and in my case Object "Change" and used a correct Name and now the Dropdown is working fine.
Thanks to Florian Schmidt. He gave tne advice to check for filter on the related attribute.
When i drop the filter, the drop down menue is working again. Nevertheless the filter is on purpose and i need it. This looks like a bug.
You are welcome!
But my first name is Fabian not Florian.