I have tested this issue on IE versions 7 and 8, Chrome and FireFox and can produce the issue on all. The machine running the app is running Windows 7 with IIS version 7.5 and is using .NET Framework 3.5.

 

I also noticed your example doesnt use a .master file and the ScriptManager should be contained within it.

 

 

Additionally, I have noticed that when using FF and Chrome, if I cancel an upload I also get this error.

 

Regards

 

Paul