Fix for the rsErrorOpeningConnection in SSRS 2016

The error, rsErrorOpeningConnection has been asked many times in almost
all versions of SQL Server Reporting Services starting from SSRS 2008. Various
reasons have been given. The error message hints at what may be wrong.

In the present case, a simple report with embedded data source  was created in Report Builder 3 by the creator owner who is also the local administrator. This report was saved to the
Report Server (also gets uploaded to the Web Portal front-end).

The report is processed in the Report Builder without any error.

If the Where_2 report is double clicked this is the message displayed after correct authentication
by the local administrator.

The Data Source is shown in the following and the report gets correctly displayed in
the Report Builder and it can be saved to the Report Server as seen above.

The rsErrorOpeningConnection arises because of the credentials to the Data Source is not
properly configured (what is shown is the default). Review the credentials for the data source:

When you make the proper entries for the report as shown here:

The error goes away.

 The SQL Server 2016 SP1, the Report Builder 3 are both on a Dell Laptop with Windows 10 Pro. The Reporting Services configuration is for Native Mode managed by the laptop owner.