ReportViewer hangs on 2008R2 server after SP1

We've got a couple of ASP.net application on a 2008R2 machine. After upgrading this box to 2008R2 SP1 one certain report stopped working. There is a message: 3005 in the event log: Report could not be loaded. The weird thing is that if I replace the report by another one everything is fine. All other reports are running. When I put this report in the CR2011 Application it runs fine...

Any hints?

Thanks, Oliver