I hit the following error on a simple page under ASP.NET 2.0 which mostly contained a single Datagrid feed by an SqlDataSource. This site was on my local machine and not a web farm.
After spending hours digging around the net and not finding the answers, I started playing around and found that at some point in time, the ViewState for the control had been set to "false". Cleared that value and poof, worked like a champ.
While I probably should have checked for that since it said it was an invalid ViewState, but I did not figure it was really a "missing" ViewState :)
Here is a bit of the original error message:
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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.
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: <a large value I removed for this post>.]
[HttpException (0x80004005): 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.]
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
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