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.