Operation is not valid due to the current state of the object, when I select a dropdown list

I have radcombo boxes on aspx page, and when I select any option from then it gives error`Server Error in '/' Application.

Operation is not valid due to the current state of the object. 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.InvalidOperationException: Operation is not valid due to the current state of the object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: Operation is not valid due to the current state of the object.]
System.Web.HttpValueCollection.ThrowIfMaxHttpCollectionKeysExceeded() +2692302 System.Web.HttpValueCollection.FillFromEncodedBytes(Byte[] bytes, Encoding encoding) +61
System.Web.HttpRequest.FillInFormCollection() +148

[HttpException (0x80004005): The URL-encoded form data is not valid.] System.Web.HttpRequest.FillInFormCollection() +206
System.Web.HttpRequest.get_Form() +68
System.Web.HttpRequest.get_HasForm() +8735447
System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull) +97 System.Web.UI.Page.DeterminePostBackMode() +63 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +133

Version Information: Microsoft .NET Framework Version:2.0.50727.3634; ASP.NET Version:2.0.50727.3634 `

From http://codecorner.galanter.net/2012/06/04/solution-for-operation-is-not-valid-due-to-the-current-state-of-the-object-error/

Issue happens because Microsoft Security Update MS11-100 limits number of keys in Forms collection during HTTP POST request. To alleviate this problem you need to increase that number.

This can be done in your application Web.Config in the <appSettings> section (create the section directly under <configuration> if it doesn’t exist). Add 2 lines similar to the lines below to the section:

<add key="aspnet:MaxHttpCollectionKeys" value="2000" />
<add key="aspnet:MaxJsonDeserializerMembers" value="2000" />

The above example set the limit to 2000 keys. This will lift the limitation and the error should go away.