Welcome to the MSDN newsgroup Chris,

As for the custom authentications like Forms Authenticaiton, it require the
client-side application interactively provide the username/password
credentials, therefore when report server is configured to use forms
authentication, we can access the reports or other resources through
browser. For those client application which programmatically access the
reports, such as the ReportBuilder tool, it will fail to provide user
credential interactively, so this is one of the limitation when using forms
authentication currenlty.

Regards,

Steven Cheng
Microsoft Online Support

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