Try this in your code where you call WorkListItem.Update() :
Dim MyK2Conn As New K2ROM.Connection
Dim configurationAppSettings As System.Configuration.AppSettingsReader = New System.Configuration.AppSettingsReader
Dim K2Server As String = CType(configurationAppSettings.GetValue("PlanServer", GetType(System.String)), String)
Dim K2ConnStr As String = CType(configurationAppSettings.GetValue("K2ConnectionString", GetType(System.String)), String)
Dim MyWLItem As K2ROM.WorklistItem
Try
MyK2Conn.Open(K2Server)
MyWLItem = MyK2Conn.OpenWorklistItem(Request.QueryString("sn").ToString, "ASP")
..... do whatever you want.....
MyWLItem.Update()
Catch ex As Exception
' replace 9999 with the 4-digits error code you recieved
If ex.Message.IndexOf("9999") = 0 Then
' Handle the exception
End If
End Try