The following error occurred when trying to run reports:
An error has occurred during report processing. (rsProcessingAborted) Cannot create a connection to data source 'DataSource'. (rsErrorOpeningConnection) Login failed for user 'domain\service_account'.
When you are creating the new database connection, on the last screen, there is a check box at the bottom for "Use as Windows credentials when connecting to the data source".
This setting needs to be enabled. If it was not enabled when you created the database/connection, then edit your connection and enable it. (see screen shot attached)
Alternatively, you can go directly to go to the datasource on the SQL Reporting Services server (i.e. http://server_name/reports) and select the option:
"Use as Windows credentials when connecting to the data source"