Hi Jens, since function elements also have a protocol.xml file that was generated through the function definition, that would be the first place to look. You say you checked the column parameter, but have you checked if the table parameter is correct? How does the table parameter expose the column you are looking for?

At this point, there might still be a problem in how the function definition is exporting these, how the server code is building the function's protocol, or how the UI is interpreting it. If the protocol should be fine, perhaps check the slnet communication between client and server to isolate the fault to one or the other.