A user has been added to a SSRS role for the report folder or report but is still prompted to login. This can happen if Internet Explorer isn't configured to persist the user's Windows login to Intranet sites.
To rectify this, in Internet Explorer open Tools > Internet Options > Security tab
Change the settings for the Local Intranet Zone by clicking Custom Level
Near the bottom of the list of settings under User Authentication, select the radio
button for the option titled:
Automatic logon with current user name and password
Click OK to close and save the settings and then restart IE.
Weblog by Paul Turley and SQL Server BI Blog.