i’m wondering what i’m doing wrong

as soon, as i add an UPDATEPROCESSDATA Action
using <WF_PROCESS_INST_ID> as Input to Parameter REQUEST_ID
im getting following error wile executing

Workflow error An error occurred while completing the action. Verify if the action has been completed. If not, then try again.
Stack Trace:System.MissingMethodException: Method not found: '!!0[] System.Array.Empty()'. at Advantys.Workflow.Engine.WorkflowsManager.ActivityInstanceComplete(Int32 processInstanceId, Int32 activityInstanceId, WorkflowUserIdentity connectedUserIdentity, WorkflowUserIdentity requesterUserIdentity) at Advantys.Workflow.Web.UI.Portal.ApplicationPages.ApplicationComplete(PageController pageController, KeyedList wfgParams)

for those who are happening to get the same error, the solution was to install .net 4.6 on the server.