Unable to access ArcGIS Enterprise sites using ArcGIS Web Adaptor URL

Description

When attempting to access an ArcGIS Enterprise portal and ArcGIS Server sites using the ArcGIS Web Adaptor URL, the following error message is returned:

Error:   
HTTP Error 503. The service is unavailable

Cause

The issue occurs when the ArcGIS Web Adaptor application pool in the Application Pools list of Internet Information Services (IIS) is stopped, or configured incorrectly through the configuration page or the command line utility from the machine hosting the Web Adaptor. Refer to ArcGIS Enterprise: Configure ArcGIS Web Adaptor for more information.

Solution or Workaround

To resolve the issue, restart the ArcGIS Web Adaptor application pool in Internet Information Systems (IIS) Manager. Refer to Step 1 of Microsoft: How to create a new application pool for the guide to open the IIS Manager depending on the machine's operating system. In Application Pools, click Restart on the Actions pane.

If the issue persists, ensure that the ApplicationPoolIdentity identity type is selected for the ArcGIS Web Adaptor application pool in the Application Pool Identity window, as shown in the image below. Refer to Microsoft: Configuring IIS Application Pool Identities for more information.