Problems with WebForm_DoPostBackWithOptions after installing .NET4.5

We installed .NET 4.5 on a server with .NET 4.0 Asp.Net web applications. New .NET 4.5 applications work ok, but existing 4.0 webform applications have a problem: http post request with WebForm_DoPostBackWithOptions don't work anymore. If the requests are traced with Fiddler, Fiddler reports that the request body is empty. Something is going wrong. The weird thing is that the problem only occurs when using  IE, and not with Chrome or FireFox. Any ideas? Upgrading the 4.0 applications to 4.5 doesn't help either.

Thanks,

Bert-Jan