Here is a bit of the original error message:
Server Error in '/' Application.
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.
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 configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
Source Error:
[No relevant source lines]
Source File: c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\c7312a20\a7ad63b1\App_Web_anemjuum.0.cs Line: 0
Stack Trace:
[HttpException (0x80004005): Unable to validate data.]
System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +2546796
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +214
[ViewStateException: Invalid viewstate.
Client IP: 127.0.0.1
Port: 6129
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.0.4) Gecko/20060508 Firefox/1.5.0.4
ViewState: .]
[HttpException (0x80004005): 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.]
System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +116
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +251
System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
System.Web.UI.HiddenFieldPageStatePersister.Load() +220
System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +83
System.Web.UI.Page.LoadAllState() +35
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6953
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +154
System.Web.UI.Page.ProcessRequest() +86
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +18
System.Web.UI.Page.ProcessRequest(HttpContext context) +49
ASP.emk_stats_viewrecentposts_aspx.ProcessRequest(HttpContext context) in c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\c7312a20\a7ad63b1\App_Web_anemjuum.0.cs:0
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +154
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64
Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.42
The number of hours I spent on this error, browsing the net, trying to find answers. The page is contains a simple grid and uses a SqlDataSource, very simple stuff! The site was on my local machine and no web farm involved!
The end result of this was also very simple, it appears at some time the grid had its EnableViewState toggled to false. Yep, that is all I needed to change, removed that value and presto, worked like it should. Guess I should have looked at that in the beginning, but I did not ever remembe turning it off. The error message was pointing me to an invalid ViewState but I did not think about a “missing” one...
Note: You may also receive this error if a page is loading and you have a large ViewState and before the page finishes, someone clicks on an option of the grid (like a command button such as Delete, Update, Select). Since the ViewState had not yet completely loaded, it would give the above error message on the postback.
At least all this work tracking down the problem should ensure I do not fall for it again :)