[HttpException (0x80004005): Impossibile convalidare i dati.]
   System.Web.Configuration.MachineKey.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +195
   System.Web.UI.LosFormatter.Deserialize(String input) +60

[HttpException (0x80004005): Authentication of viewstate failed.  
1) If this is a cluster, edit <machineKey> configuration so all servers use the same validationKey and validation algorithm.  AutoGenerate cannot be used in a cluster.  
2) Viewstate can only be posted back to the same page.  
3) The viewstate for this page might be corrupted.]
   System.Web.UI.LosFormatter.Deserialize(String input) +118
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +101
..... ecc. ecc.