If you have an application that you are trying to test locally (such as on a Vagrant VM) or on a development server that has a self-signed certificate, Behat will probably complain of an SSL certificate problem because of an invalid certificate chain (GuzzleHttp\Exception\RequestException)
This is because Guzzle (the http client used by Goutte, the default Mink driver) believes the connection to be insecure, which technically it is, but for testing purposes we can ignore this.
If your application (or stack) is configured for https only at all times this means you can’t switch to http to test, since that would be testing configuration that is not representative of your live environment. To circumvent this you can disable SSL verification for cURL in Guzzle.