The problem is most probably related to the data. Please test to run the design preview of the report with the same set of records producing the error in the viewer, in order to check if this is the problem. You can also subscribe for the report's Error event and log the exception.
If you have chance, test also to upgrade the project or only the reports to the latest available Telerik Reporting Q2 2014 SP1 version 126.96.36.1994.
In case you decide to upgrade all projects after testing the reports, keep in mind that as of Q1 2014 the WPF/Silverlight ReportViewer uses implicit styles instead of the StyleManager to apply themes.
The latest available Telerik Reporting release Q2 2014 SP1 version 8.1 14.804 is built against UI for WPF Q2 2014 SP1 version 2014.2.729.40. Thus you will need to download and the corresponding UI for WPF official or a greater version. If you use greater version of Telerik UI for WPF, you will need to apply proper binding redirects and update the corresponding XAML resources for the WPF controls.
Details how to add a WPF ReportViewer are available in the How to: Add report viewer to a WPF application help article. Also the following blog post elaborates on all strategies for using the WPF ReportViewer with the existing UI for WPF assemblies (NoXAML and full): Setting a theme of the WPF/Silverlight Report Viewer (Q1 2014 and later).
Feel free to open a support ticket and send us the problematic report and test data, in order to run the report and reproduce the error.
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