Why I am getting a windows security Pop Up on accessing aspx pages with in a report folder in an asp.net application in IIS 7.5?

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C#2.0

C#

ASP.NET

IIS7.5

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Why I am getting a windows security Pop Up on accessing aspx pages with in a report folder in an asp.net application in IIS 7.5?

All aspx pages under other folders and subfolders are coming fine. But on a report folder even with permissions set and even on removal of read only property, its not working.

A windows security box pops and on entering the credentials "resource not found error" comes and afterwards "the resource not found error" comes under all Report folders.