If I run this code from a Workstation that is in the same domain as the server workflow it works perfectly.
Now if the Workstation is in a workgroup that is broken and stops at the last statement in this code snippet.
   Dim app As WorkflowApplication = New WorkflowApplication
                 app.Registration = New WorkflowServerRegistration (serverName)
                 app.ApplicationName = "Controle_ouverture_dr_client"
                 app.ApplicationType = WorkflowApplicationType.Client
                 Dim parameters As Dictionary (Of String, String) = New Dictionary (Of String, String)
                 parameters.Add ("LF_ID" nID_LF)

                 Dim LF_Conn As WorkflowConnection = app.Open