No Test Connection Message in A+

I've configured all other settings for services and providers and am attempting to connect Google Calendar.

When clicking the Check Requirements link, I get
[Appointments+] Your server installation meets requirements.

After completing all of the other steps, saving the settings, and clicking "Test Connection," I get absolutely no message at all; no failure or success.

I have checked the Logs, and see a (503) Backend Error message.

Am I overlooking something obvious?