Using the Connections Mac Plugin with an Kerberos enabled Connections environment, you need to customize your existing Kerberos configuration to get the plugin working without any problems.
The Plugin is using the URL /files/serviceconfigs to get some environment information. Because the plugin cannot use Kerberos, WebSphere will send back the configured fallback URL (/files/serviceconfigs?noSPNEGO). This URL refers to a local html file on the webserver which do some redirect stuff. The plugin can’t understand the redirects and uses the ?noSPENGO html page instead of the /files/serviceconfigs xml. Of course this brakes all functionality.
To solve this issue just add another exclusion (/files/serviceconfigs) to your Kerberos configuration. After this the plugin will work as expected.