Check if you have added requestValidationMode setting to your httpRuntime web.config's settings, like described in this tracker entry here. It may also be a good idea for you to test the updated sample in that tracker entry to see if it's working for you or not. You can also find more info on our ReportViewer page here.
Hmmm... here I'm assuming you have a VS2010, so please correct me if I'm wrong on that one.
If this doesn't help, please create the most minimal reproducing sample application, clean it, zip it and then email it to the support email address (see my signature) along with the full Http reference to this thread. Please also post here when you have sent it.
Hope this helps,