After installing the ASP.NET ValidatePath Module, Reporting Services will give you the following error:
Server Error in ‘/ReportServer’ Application.
Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application’s trust level in the configuration file.
Exception Details: System.Security.SecurityException: Request for the permission of type System.Web.AspNetHostingPermission, System, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 failed.
Microsoft has posted KB article #887787 with revised Reporting Services .config files that fixes the issue.