[C#]Validation of viewstate MAC failed

[Jackons]

Salve a tutti,
Ho un problema con il mio sito web.
Da qualche giorno quando si tenta di accedere nell'area riservata del sito tramite form di login si presenta questo errore su alcuni dispositivi (windows xp e android no windows 7):

Server Error in '/' Application.

Validation of viewstate MAC failed. If this application is hosted by a
Web Farm or cluster, ensure that <machineKey> configuration specifies
the same validationKey and validation algorithm. AutoGenerate cannot
be used in a cluster.

Description: An unhandled exception occurred during the execution of
the current web request. Please review the stack trace for more
information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Validation of viewstate
MAC failed. If this application is hosted by a Web Farm or cluster,
ensure that <machineKey> configuration specifies the same
validationKey and validation algorithm. AutoGenerate cannot be used in
a cluster.

Qualcuno sa come posso risolvere? Grazie