Issue accessing ArcGIS Server services through an ArcGIS Web Adaptor via the HTTPS protocol

Description

Accessing a server manager through a public facing ArcGIS Web Adaptor instance returns the following error message:

Error:   
HTTP Error 403.16 - Forbidden: Your client certificate is either not trusted or is invalid.

Cause

This is an issue with the SSL certificate exchange with ArcGIS Enterprise, and occurs when 'Client certificates' is set to 'Accept' in SSL Settings of the ArcGIS Web Adaptor instance. By default, this is set to 'Ignore' on the website. Login errors, authentication errors, and '403 Forbidden' errors are returned as authentication information is not sent from the client side when accessing public facing services.

Solution or Workaround

Follow the steps listed below to resolve the issue:

1. Launch Internet Information Services (IIS) Manager.
2. Expand Default Web Site in the left pane and select the <arcgis> (Web Adaptor instance).
3. Double-click SSL Settings.
4. Select Ignore under Client certificates.