Greg ,

 

1 - when the file be uploaded , it will do the postback , and fire the FileUploaded event.

  if your page use MSAJAX, it will be a AJAX postback , otherwise it will be a FORM postback.

  you can use javascript to catch the PostBack event http://ajaxuploader.com/document/scr/JavaScript-API.htm

 

2 - Uploader should work fine for UpdatePanel .

 I think your code should be something special , for example ,maybe dynamic creating the control.

 

You can try to build aother simple page with your control creating logic , and try to reproduce this error.

 

And then post the code , I will help you to check which part cause that error.

 

Regards,
Terry