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

Hi All,
In pretty much all of the pages in our website, and on regular occasions, we are experiencing the error:
"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."
Do you know what I can do about this please?
Thanks in advance,
Ryan, UK
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