Problem coding using OpenProcessInstance

Hi All, first thanks for reading this.

I have a problem, i need to be able to change a DataField value of a K2 BlackPearl process, from the outside, for example from a personal webservice.

I will then use the DataField value has a trigger to continue a process activity.

But i keep stepping on a Error, i get a Object reference not found with the K2Con.OpenProcessInstance(ps.ID) method, i know that the ps.ID is ok, i can list the processes but that is about it

Here is the code

Thanks you in Advanced

string hostServer = System.Configuration.ConfigurationManager.AppSettings["K2HostServer"].ToString();

 //conenct to the K2 host server to get the process list
WorkflowManagementServer wms = new WorkflowManagementServer(hostServer, 5555);

 wms.Open();
 //grab list of processes within the server
ProcessInstances processInstances = wms.GetProcessInstances();

// connect  to the k2 client interface to be able to change datafields
Connection K2Con = new Connection();
try
{
  foreach (SourceCode.Workflow.Management.ProcessInstance ps in processInstances)
  {
                            
     // connect to the specific process using the
    SourceCode.Workflow.Client.ProcessInstance myProcessIntance = K2Con.OpenProcessInstance(ps.ID);                      

   // look for the Process with the SGD number
   if (myProcessIntance.DataFields["sNumProcesso"].Value.ToString() == NumProcesso)
   {
    myProcessIntance.DataFields["myDataFieldName"].Value = "mynewvalue";
    myProcessIntance.Update();
   returnResult = true;
     break; // no point in continuing, since we found the process
   }
  }
 }
  finally
   {
      // close the connection
     K2Con.Close();
    }
  }
  catch (Exception ex)
   {
                System.Console.WriteLine("Error updating process datafield, exception ")
            }