502 Web server received an invalid response while acting as a gateway or proxy server

Error Message

Accessing a web server with not trusted or expired Portal for ArcGIS and/or ArcGIS Server self-signed certificates returns the following error, even if the web server certificate is valid.

Error:   
Error: 502 Web server received an invalid response while acting as a gateway or proxy server

Cause

A certificate is a digital file that contains information about the identity of the web server. It also contains the encryption technique to use when establishing a secure channel between the web server and the organization. See: Enable HTTPS on your web server

Despite a valid Secure Sockets Layer (SSL) certificate on the web server, the error is returned because the Portal for ArcGIS and ArcGIS Server self-signed certificates have expired.

Note:
There are no web adaptors in an Azure Web Geographical Information Systems Cloud Builder deployment. Instead, a URL Rewrite is used, redirecting from a web server URL to HTTPS port 7443 for Portal for ArcGIS, and HTTPS port 6443 for ArcGIS Server.

Solution or Workaround

1. Export the self-signed certificates from Portal for ArcGIS and ArcGIS Server.
2. Install these certificates to the server's trusted storage.
3. Switch Portal for ArcGIS and ArcGIS Server to use their self-signed certificates.