Cannot create a connection to data source Error (rsErrorOpeningConnection) in SSRS
First thing I would try is to get a bit more information on the error - that's a pretty generic message.
You could enable remote errors as per the error message and replicate the error for more information:
Enable remote errors.
Or check the Report Server error logs to see what error was logged.
%programfiles%\Microsoft SQL Server\<SQL Server Instance>\Reporting Services\LogFiles\
The next step would be to connect as the Data Source user to the database, run any code/stored procedures that the report is using with the same parameters you're using when running the report, and see if any errors occur.
In SQL Server 2008 in addition to the above two options you have a third option to make this setting through SQL Server Management Studio.
1.Start Management Studio and connect to Report Server Instance (make sure you select 'Reporting Services' server type).
2.Right click on the ReportServer and Select Properties
3.Click Advanced
4.In EnableRemoteErrors, select True.
5.Click OK.
I had the same issue "Cannot create a connection to data source...Login failed for user.." on Windows 8.1, SQL Server 2014 Developer Edition and Visual Studio 2013 Pro. All solutions offered above by other Stackoverflow Community members did not work for me.
So, I did the next steps (running all Windows applications as Administrator):
VS2013 SSRS: I converted my Data Source to Shared Data Source (.rds) with Windows Authentication (Integrated Security) on the Right Pane "Solution Explorer".
Original (non-shared) Data Source (on the Left Pane "Report Data") got "Don't Use Credentials".
On the Project Properties, I set for "Deployment" "Overwrite DataSources" to "True" and redeployed the Project.
After that, I could run my report without further requirements to enter Credentials. All Shared DataSources were deployed in a separate Directory "DataSources".
