Reporting Services in SQL Server 2005 runs on IIS 7.0. After configuring Reporting Service, two virtual directories will be created by default their Application pool – DefaultAppPool.
Every Application pool is set to pipeline mode and there are two types of Pipeline modes namely, Integrated and Classic.
If the Pipeline mode of the Application Pool attached to the virtual servers is Integrated you will get the above error and you will change the Integrated mode to Classic mode in IIS. This will not require a restart for either IIS or SQL Server Reporting Services.
There is one other method to resolve this – create a new application pool with Classic mode and attach that Application pool to both virtual servers from the Reporting Server Configuration Tool.]]>