Server Error: Operation is not valid due to the current state of the object

This error could occur after a Windows Update that reduces the allowable size of the HTTP Context to be sent/received from the server to the clients browser.

See the Vulnerability in ASP.NET Could Allow Denial of Service Microsoft Security Advisory for more information.

The default value of MaxHttpCollectionKeys is set to 1000 by the security patch. You can override this value by adding the following key to the appsetttings section in your web form’s web.config file:

<add key="aspnet:MaxHttpCollectionKeys" value="10000" />

(10000 is just a suggestion; you can set it to the value of your liking.)