I am performing integration tests in sandbox with an internal system through web services.
Within the VA I was able to perform the GET using curl, passing the --insecure parameter.
I cannot find this parameter in the webservice http operation configuration.
I dug deeper into the problem. I noticed that when the test connection is made, the request also sends a Content-Type: application/json header, even though no header is configured in the request.
It turns out that the application does not accept this header because it is just a get to list users.