GPO: SharePoint Sites Keeps Asking Users to Login
Solution via GPO: Computer Configuration | Policies | Windows Settings | Security Settings | Local Policies | Security Options: Network access: Do not allow storage of credentials or .NET Passports for network authentication - set it to Disabled . link
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Read the complete post at http://msforums.ph/blogs/jayr/archive/2010/05/21/gpo-sharepoint-sites-keeps-asking-users-to-login.aspx