Credentials prompt for an aspx page configured for anonymous authentication

From Flex we call a Upload.aspx page which is configured for anonymous authentication. Most of the time, it works like a charm, but once in a while, the browser does prompt a enter credentials popup...

The whole site is configured for Windows Authentication, but some pages and folders are set to use anonymous authentication. This is done using the location tag in web.config.

What could be开发者_Python百科 the reason for this?

UPDATE: Only happening in Internet Explorer... they should deport it.

If you use FireFox with FireBug, open up the Net panel, it will show you the request making the permissions request. If you cancel it, it will show an access denied in red, and that will easily help you hunt the issue down.

Maybe it's because a file (image, css, etc.) is being referenced that doesn't exist?

A couple of things I would check out:

1. Are there any images/other files that are added to your page using the FQDN? If so, are any of these pointing to an external site or staging site that might require credentials?

2. Is it possible that someone has removed permissions from the application pool credentials on the web server for some specific file or files the site is requesting?

Is the site load balanced or part of a farm? It could be that one or many servers are configured incorrectly, and the rest are ok. Then if by chance you hit the bad servers, you could get the prompt.