An error occurred while determining the media type of a file viewer.

Hello.

Any idea why some of our users can't download PDF document from our site?

Here is an error from the log:

UnknownException: An error occurred while determining the media type of a file viewer.  
See inner exception for details. --->
System.InvalidCastException: Unable to cast object
of type 'System.Collections.Generic.List`1[Telligent.Evolution.DocumentPreview.DocumentViewerConfiguration]'
to type 'System.Collections.Generic.IEnumerable`1[Telligent.Evolution.DocumentPreview.DocumentViewerConfiguration]'. at Telligent.Evolution.Components.PluginManager.GetIEnumerableOfTFromCache[T]() at Telligent.Evolution.Components.PluginManager.Get[T]() at Telligent.Evolution.DocumentPreview.DocumentViewerConfiguration.get_IsEnabled() at Telligent.Evolution.DocumentPreview.DocumentViewerConfiguration.GetMediaType(ICentralizedFile file, IFileViewerOptions options) at Telligent.Evolution.Components.FileViewerService.GetMediaType(ICentralizedFile file, FileViewerViewType viewType, Boolean outputIsPersisted) --- End of inner exception stack trace --- at Telligent.Evolution.Platform.Logging.ErrorLogger.Unknown(String internalMessage, Exception innerException) at Telligent.Evolution.Components.FileViewerService.GetMediaType(ICentralizedFile file, FileViewerViewType viewType, Boolean outputIsPersisted) at Telligent.Evolution.MediaGalleries.Plugins.GalleryApplicationType.•(PageContext , IPageOutputController –, Boolean ‚Š) at Telligent.Evolution.Urls.Routing.SiteUrlRouteHandler.GetPageHttpHandler(RequestContext requestContext, PageContext pageContext) at Telligent.Evolution.Urls.Routing.SiteUrlRouteHandler.GetHttpHandler(RequestContext requestContext) at System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(HttpContextBase context) at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) at System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error) at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb) at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus) at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags) at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)

Thanks.