Are you using SQL Server 2005?
If so, make sure they are all installed on the server hosting the reporting services. (Also, if you ever have a site with the .NET reportviewer control, it will also need these fixes) Link Also make sure that all the usual security fixes apply to your SQL server, namely this. Link and this. Link
This may fix your problem. All updates were distributed in the Report Viewer 2005 Redistributable Service Pack 1 (SP1) Redistributable package, which fixed several uncomfortable errors that I encountered.
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