Symptoms

SmartForms Workflow Designer (Silverlight) fails to load: Unable to load plug-in: :Async_ExceptionOccured] Arguments: Debugging resource strings are unavailable
 

Diagnoses

The MIME '.bin' extension was disallowed in IIS which caused a 404 (seen using Fiddler) when loading the application.
 

Resolution

Make sure that when using Fiddler to troubleshoot SilverLight load failures that the corresponding .bin file is downloaded when opening the link in a browser. If a link like below does not attempt to download the file, there is most likely not a '.bin' MIME type or the extension is blocked.

http:///designer/Workflow/ClientBin/SourceCode.WebDesigner.DesignerPlugin.General.4.15020.15057.1.bin