Formgrids error...

I keep getting this screen in forumgrids. How can I fix it?

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 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:

The source code that generated this unhandled exception can only be shown when compiled in debug mode. To enable this, please follow one of the below steps, then request the URL:

1. Add a "Debug=true" directive at the top of the file that generated the error. Example:



or:

2) Add the following section to the configuration file of your application:







Note that this second technique will cause all files within a given application to be compiled in debug mode. The first technique will cause only that particular file to be compiled in debug mode.

Important: Running applications in debug mode does incur a memory/performance overhead. You should make sure that an application has debugging disabled before deploying into production scenario.

Stack Trace:


[ViewStateException: Invalid viewstate.
Client IP: 41.132.215.189
Port: 54246
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.0; Trident/5.0)
ViewState: 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...]

[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) +106
System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +14
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +237
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() +207
System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +105
System.Web.UI.Page.LoadAllState() +43
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
System.Web.UI.Page.ProcessRequest() +80
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
System.Web.UI.Page.ProcessRequest(HttpContext context) +49
ASP.subscriber_fgraceindexb_aspx.ProcessRequest(HttpContext context) +4
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

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Version Information: Microsoft .NET Framework Version:2.0.50727.4206; ASP.NET Version:2.0.50727.4209

Happens to me at night sometimes,not sure if its when they are updating formlines?

I don't know much, but you can try...
.NET Framework version may be outdated.
Clear your Browser cache.
Try another browser eg Firefox/Chrome etc and see if it does the same.
Browser up to date?

Hope you come right.