Sometimes after reinstalling rendering extension there are still old versions of some dll files in the list.
Probably you also faced with this error:
"Unable to find assembly 'PerpetuumSoft.ReportingServices.Viewer.ExtensionLink, Version=188.8.131.52, ...'",
where 184.108.40.206 is the previous version of the dll file.
First of all uninstall rendering extension, clear the cache, then install them. Restarting SQL Server and Reporting Services clears the cache sufficiently.
Make sure that they all have the necessary version in the '\Microsoft SQL Server\MSRS10_50.SQLSERVER_2008R2\Reporting Services\ReportServer\bin\' directory,
where MSRS10_50.SQLSERVER_2008R2 is the name of your directory with Reporting Services.
If there are still old dll files in that directory, replace them by dll files from this directory: '..\Perpetuum Software\Reporting Services 2008 Rendering Extension for Silverlight Viewer\':
Restart Reporting Services.
Find more information in this article: How to add Rendering Extension to Reporting Services manually.