The following is a snapshot of the error that you might see if the Reporting Services data sources are not updated when the account is changed or updated:
SQL Server Reporting Services
Most of us are probably already aware of this, but this question comes up quite often. If your Reporting Services service account has changed because you moved your Team Foundation Server installation or if you are using a different account, you will need to run TfsAdminUtil ChangeAccount.
In addition to this, you will also need to update the password and/or username for the TfsReportDS and the TfsOlapReportDS data sources manually using the Reporting Services web site at: http://<ATMachineName>/Reports/
More documentation on this topic: http://msdn2.microsoft.com/en-us/library/ms252480(VS.80).aspx