HTTP Error 401.3 - Unauthorized

HTTP Error 401.3 - Unauthorized
You do not have permission to view this directory or page because of
the access control list (ACL) configuration or encryption settings for
this resource on the Web server.

We have a Web application with Forms Authentication, and any export from the website

resulted in above error.

In the beginning, all we got was generic 401 - Unauthorized acess

I had to go to IIS,Default Web Site -> Error Pages ->Edit Feature Settings -> and check 

"Detailed errors" and NOT  "Detailed errors for local request...."

Once that's done, I got this instead of generic 401, I got 401.3

To resolve this

1) I checked my application pool, it was using Application Pool Identity

2) So, on my web site, I clicked on Anonymous Authentication (make sure that its enabled),

Edit, change the identity from specific user to "Application Poool Identity"

3) IIS Reset, and no more 401 Errors

Hope this helps...