Uninstalled and reinstalled what was supposed to be a "developer mode" of Silverlight.  After updating this "latest version" from v3 to v4, tried again, and got this almost identical error:

 

INFO ScriptVersion ,20110321-01

INFO UploadType/addontype,Auto/Silverlight

BEGIN event initialize

END event initialize

BEGIN event browse

END event browse,true

BEGIN event silverlight select

END event silverlight select,true

BEGIN event start

END event start

BEGIN event taskstart,{DAN4.XML:4179991}

END event taskstart

INFO InitFrame 

ERROR silverlight report,error,de712d1c-810c-42eb-aaa2-55faa77e4a04,io,WebException : UnknownError,[HttpWebRequest_WebException_RemoteServer]

Arguments: NotFound

Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem. See http://go.microsoft.com/fwlink/?linkid=106663&Version=4.0.60310.0&File=System.Windows.dll&Key=HttpWebRequest_WebException_RemoteServer

INFO Change to Silverlight Load Mode

INFO silverlight start load mode ,720896/4179991

BEGIN event error,DAN4.XML:SLM error:0,

END event error,true

BEGIN event taskerror,error:SLM error:0,

BEGIN event taskerror

BEGIN event stop

END event stop

 

 The only difference is that it reached 17% before crapping out, and the GUID in the first error message is now different?

 

Also, please note that it is taking an Incredibly long amount of time to get this far - from starting to final error is 22 minutes!