I was getting the above error while running one of the custom SSRS report inside CRM 2011.
To resolve this issue I had to follow these steps
- Start SQL Server Management Studio
- Expand Security, then expand Logins
- Select and right click the account under which the SQL Server Reporting Services is running.
- Select User Mapping and select YouOrg_MSCRM database and specify following role membership
Source: Nishant Rana’s Weblog