OK - loaded the sample on our server.  The files queue OK, but then when hitting submit is when the issue occurs (see screenshot attached). It doesn't matter if we do it via drag-drop or browse.  On my local machine I successfully get the postback message and see the ListBox updated.

We're using IIS 7 in Classic mode. Although I've created a new site (tried .NET 2 versus 4, Classic vs Integrated) just for the samples and still get the error.  It's so strange because it doesn't happen every time, I can drag 10 files, all at once or individually, and click Submit and it works fine.  Then I'll drag another file and it hangs after the submit button click.

Our browser is IE v11.0.9600.18282.  I should mention we're running the browser in a Ctrix environment, but I'm not sure why that would have any effect.

I welcome any suggestions!  Thank you.