add error info

Author: greg-lindsay

articles/application-gateway/end-to-end-ssl-portal.md

@@ -6,7 +6,7 @@ services: application-gateway
 author: greg-lindsay
 ms.service: application-gateway
 ms.topic: how-to
-ms.date: 11/14/2019
+ms.date: 03/28/2023
 ms.author: greglin
 ms.custom: mvc
 ---
@@ -26,6 +26,9 @@ To configure end-to-end TLS with an application gateway, you need a certificate
 
 For end-to-end TLS encryption, the right backend servers must be allowed in the application gateway. To allow this access, upload the public certificate of the backend servers, also known as Authentication Certificates (v1) or Trusted Root Certificates (v2), to the application gateway. Adding the certificate ensures that the application gateway communicates only with known backend instances. This configuration further secures end-to-end communication.
 
+> [!IMPORTANT]
+> If you receive an error message that the "Backend server certificate is not whitelisted with Application Gateway," verify that the frontend certificate Common Name (CN) matches the backend certificate CN.
+
 To learn more, see [Overview of TLS termination and end to end TLS with Application Gateway](./ssl-overview.md).
 
 ## Create a new application gateway with end-to-end TLS