Hi,

I hope someone will be able to help me with this.

We have recently migrated all of our clients' sites to a new server running IIS7 - all the sites were originally running on a server running IIS6.

Ever since the migration, lots of our clients are reporting error messages. There seems to be quite a number of issues related to sending emails and also, we have had the following error message reported by several different clients:

    Server Error in '/' Application.
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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.

I have read elsewhere that this error can appear if a button is clicked before the whole page has finished loading. But as this error has now appeared on multiple sites and only since the server migration, it seems to me that it must be something else.

I was wondering if someone could tell me if there is something specific which needs to be changed for .NET sites when sites are moved from a server running IIS6 to a server running IIS7? I don't deal with the actual servers very much so I'm afraid this is very much a grey area for me.

Any help would be very much appreciated.