The problem is coming from the fact that application cannot initialize the Silverlight application which is required for Sitefinity, because we use RadUpload for Silverlight and the xap cannot be found. The xap is located under http://host/ClientBin/Telerik.Sitefinity.Silverlight.xap
You can take a look at this blog post and
please verify that your server has Silverlight installed and running.
All the best,
the Telerik team
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