Ok so it turns out that the Account (Usually RTCReportPack) you set up when you publish the Reporting Services reports needs to have the ‘Allow Log on Locally’ user right on the SQL server.

If not you will see this error:

An error has occurred during report processing. (rsProcessingAborted)
Cannot impersonate user for data source ‘CDRDB. (rsErrorImpersonatingUser)
Logon failed. (rsLogonFailed)
For more information about this error navigate to the report server on the local server machine, or enable remote errors.

And make sure that it has Password never expires as well if that is ok with your service account security policy that is.