I am having the same issue..
my silverlight upload control displays properly, and lets me select files to upload. Then when I click on upload. It crashes with the error that you mentioned.
Then. IE gives the following Error:
Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; FDM)
Timestamp: Wed, 7 Oct 2009 21:52:34 UTC
Message: Unhandled Error in Silverlight Application
Code: 4004
Category: ManagedRuntimeError
Message: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.ThrowHelper.ThrowKeyNotFoundException()
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at ComponentArt.Silverlight.UI.Data.UploaderModule.InitialResponseCallback()
at ComponentArt.Silverlight.UI.Data.UploaderModule.StartUpload(String tag)
at ComponentArt.Silverlight.UI.Data.UploadItem.StartUpload(String tag)
at ComponentArt.Silverlight.UI.Data.Upload.doBeginUpload()
at ComponentArt.Silverlight.UI.Data.Upload.BeginUpload()
at ComponentArt.Silverlight.UI.Data.Upload._uploadFilesButton_Click(Object sender, RoutedEventArgs e)
at System.Windows.Controls.Primitives.ButtonBase.OnClick()
at System.Windows.Controls.Button.OnClick()
at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
at System.Windows.Controls.Control.OnMouseLeftButtonUp(Control ctrl, EventArgs e)
at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, String eventName)
Line: 57
Char: 13
Code: 0
URI: http://localhost:3950/Login.aspx?ReturnUrl=%2fUploadControlTestPage1.aspx