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
<add key="aspnet:MaxHttpCollectionKeys" value="10000" />
10000 is just a suggestion; you can set it to the value of your liking.)