If you're getting an error similar to the following in your Event Log, I've found a solution that may work for you.
NET Runtime Optimization Service (clr_optimization_v2.0.50727_32) - Failed to compile: Microsoft.ReportingServices.QueryDesigners, Version=9.0.242.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91.
In my case, re-installing SQL Server 2005 SP2 didn't work. Neither did rebooting. The only thing that seemed to correct the problem was to install the Microsoft Report Viewer Redistributable 2005. I'm not really sure why that would be required on a server, but things don't always make sense.