There is a problem with custom authentication on SQL Server 2008 R2 related with bug in SQL Server.
A solution for this problem described in this Microsoft Knowledge Base article: FIX: Forms-based authentication does not work in SQL Server 2008 R2 Reporting Services.
Trouble logging in? Simply enter your email address OR username in order to reset your password.
For faster and more reliable delivery, add firstname.lastname@example.org to your trusted senders list in your email software.