I have the same problem.
On server side I have some code-behind parsing the uploaded file and storing the result in our database. This usually takes a total duration of 5 seconds. This is done in the method that's set for OnFileValidating.
I think that the return of this postback is causing the unknown result error and I cant get rid of it.
If I press CancelAllTasks button which calls uploader.cancelall() there's a very big chance the current file being uploaded is being Validated server-side. So I basically get the error everytime.
I hope this helps with reproducing the error?
Its not a real error popup, but an alert I believe and it happens in both IE as in Firefox.
I use version 3.0.0.0.