Call Web Service action returns '401 Unauthorized' error with valid credentials

When using the Call Web Service action in a SharePoint On-Premises environment, the action fails regardless of what credentials are used.

The Remote Server Returned An Error: (401) Unauthorized

Attempt to access the web service end point via the browser on all Web Front End servers.
Locate the server where the web service end point fails.
Execute one of the following steps to disable loopback check on the Web Front End server:
- Via an administrator SharePoint Management Shell window run: New-ItemProperty HKLM:SystemCurrentControlSetControlLsa -Name "DisableLoopbackCheck" -value "1" -PropertyType dword
- Manually disable loopback checking in the registry:
  1. navigate to: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa
  2. Create a DWORD32 entry for DisableLoopbackCheck
  3. Set the value to 1

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