HTTP code 401 Unauthorized: missing or bad authentication

What causes this issue?

The HTTP call ended with HTTP error code 401: Unauthorized. The given
user / password combination for the file loader HTTP destination is not
correct.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Please contact your system administrator or check why the HTTP
destination in transaction SM59 does not work properly.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.