Symptom: 

After a DNN site was moved between servers, the DNN User Reports function was generating the following error:

There was an error while executing the configured data source: There is an error in your SQL at line 0: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)

Resolution: 

The user reports functionality does not pull the database connection from the web.config, and instead has its own configuration for db server, db name, db username/password.  Those settings were not updated when the site was moved and the web.config updated to reflect new db connection info, the DNN site loaded, but the user reports function would not becuase it was still its unique copy of db connection info, which was still set to the previous server. 

Simply select the settings for the user reports module, and change them to the current/correct db connection settings.