Hi Martijn,
When a screen is set as public, other eSpaces can use it and pass input parameters.
However, lists, objects and binary data are always passed by reference.
A reference is only valid in the eSpace were it was created.
In your case, you are trying to pass parameters by reference between eSpaces.
For more information about input parameters see: http://www.outsystems.com/help/servicestudio/9.1/index.htm#t=Designing_Actions%2FAbout_Input_Parameters.htm
Regards,
Miguel Domingues