How to debug "Unknown Server Error"?

I have full control on both server side and client side.

Recently, I am using AjaxControlToolkit.AsyncFileUploader. However under some circumstances, the AsyncFileUploader triggers browser to show an alert box with the word "Unknown Server Er开发者_如何转开发ror".

I tried to locate all exceptions in Global.asax and tried all three major browsers (FF, IE, Chrome). But it is very strange that

• No server side exception can be found in method Application_Error in Global.asax
• No client side exception can be found in FireBug, IE Developer Toolbar and Chrome internal debugger.
• No error log in Window Event log

So what can I do for this Unknown Server Error?

Thanks!

I fixed the problem already. I set breakpoints on all events header and run the application. Finally I found some events is not triggered which should be triggered.

After fixing this problem, the "Unknown Server Error" is gone.

However, I still don't know how to get more information about the "Unknown Server Error".