After upgrade, API web landing page is not available (4378572)

After upgrading Identity Manager, the QBM app landing page (API server web interface) is not available.

The log reports the following error:

System.Exception: An error occurred while processing the request: /portal-name/index.html
System.IO.FileNotFoundException
File portal-name/index.html not found

Before performing an upgrade, the Web Applications had been disabled in Designer under:

Designer | Base Data | Security settings | HTML Applications.

For each of the web applications, their properties (using Expert View) need the setting "IsInActive" to be "False" for the applications to be enabled.

Ensure that QBMHtmlApp.IsInActive is false for the applications, and restart the web services for them to retrieve the updated settings.