The following error is thrown when trying to load a Silverlight viewer in ArcGIS Server 10.4:
InvalidCastException Unable to cast object of type 'System.String' to type 'System.Collections.ICollection'
Silverlight viewers are not supported in ArcGIS Server 10.4. At this time, Responder uses Silverlight viewers for all of the Map Service functionality such as Map Viewer, Customer Viewer, and Switch Order Request page. ArcGIS Server 10.4.1 resolves this issue, and we recommend that Responder users upgrade to that version. An earlier version of ArcGIS server will be needed until Responder replaces its Silverlight viewer.