I have created an External Content Type to a SQL database and an external list.   When I tried to render the data from the SQL server to the external list, I got "Unable to render the data.  If the problem persists, contact your web server administrator".

Correlation ID:ff7c1e9d-2438-101f-d148-8d0f1b259d78

The only thing I found from the logs that is related to this correlation ID is 

WcfReceiveRequest: LocalAddress: 'https://myhostname:portnumber/SecureStoreService.svc/https'

Channel: 'System.ServiceModel.Channels.ServiceChannel' 

Action: 'http://schemas.microsoft.com/sharepoint/2009/06/securestoreservice/ISecureStoreServiceApplication/GetRestrictedCredentials' MessageId: 'urn:uuid:d80bb214-b32a-4b6e-bea3-11a74ef92130'               ff7c1e9d-2438-101f-d148-8d0f1b259d78

I did come across the thread below but I don't understand why we need to change the Authentication Mode to BDC Identity whereas I have it as Impersonate Custom Identity.

https://social.msdn.microsoft.com/Forums/sharepoint/en-US/69d937e6-f4a3-40e0-b57f-67cddb4ed12e/sharepoint-2013-unable-to-render-the-data-if-the-problem-persists-contact-your-web-server?forum=sharepointcustomization

Has anyone else experienced this issue?

Hi,

As I understand, you cannot render the data from SQL database to external list and you want to know why change to BDC in SharePoint 2013.

If you change the Authentication Mode to BDC Identity, it will let all the users browse to the external list. Whenever the user browses the external list, it is the BCS AppPool account which is used to pull the data and not the Users Identity. The application pool of a SharePoint farm is a very highly privileged account. The account has enough privileges in SharePoint.

The article below is about Authenticating to Your External System. You could focus on the Revert To Self section for more detailed information about the BDC Identity. It also can work in SharePoint 2013.

http://blogs.msdn.com/b/bcs/archive/2010/03/12/authenticating-to-your-external-system.aspx#footnote_01

You could change the Authentication Mode to BDC Identity for your issue.

Best regards,

Sara Fan