Operation is not valid due to the current state of the object. System.InvalidOperationException: Operation is not valid due to the current state of the object.
at System.Web.HttpRequest.FillInFormCollection() at System.Web.HttpRequest.get_Form() at Rhino.Commons.LongConversationManager.LoadConversationFromRequest(Boolean& privateConversation) at Rhino.Commons.LongConversationManager.LoadConversation() at Rhino.Commons.HttpModules.UnitOfWorkApplication.UnitOfWorkApplication_BeginRequest(Object sender, EventArgs e) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Cause:
Microsoft recently (12-29-2011) released an update to address several serious security vulnerabilities in the .NET Framework. MS11-100 was introduced just recently that handles potential DoS attacks.  
Unfortunately the fix has also broken page POSTs with very large amounts of posted data (form fields). MS11-100 places a limit of 500 on postback items. The new default max introduced by the recent security update is 1000.  
Adding the setting key to the web-config file overcomes this limitation, as in this example increases it to 2000.