The Mingle Connector requires that basic authentication is enabled on the server, which is off by default. Please follow these instructions to enable basic authentication:
Getting started: turning on Basic Authentication
The Mingle API supports basic authentication. However it is disabled in the default configuration.
To enable basic authentication, you need set the basic_authentication_enabled configuration option to true in the Mingle data directory/config/auth_config.yml file, where Mingle data directory is the path to the mingle data directory on your installation.
E.g. basic_authentication_enabled: true