Unable to fetch calendar subscription URL

I subscribed to a team calendar within Mac Calendar, but the subscription stopped updating. Turns out the subscription URL is broken, and returns a 400 error: "The request sent by the client was syntactically incorrect.". So I went to Confluence to get a new subscription URL, but the field is blank.

Steps to reproduce:

1. Go to "My Calendars" within Confluence
2. Click "…" next to the calendar name in the right sidebar.
3. Click "Subscribe"

Expected results:

You should see an .ics subscription URL in the modal window's input field

Actual results:

• The field is blank. (See screenshot)
• The JS console reports "Failed to load resource: the server responded with a status of 500 (Internal Server Error)" for a request to "https://avvojira.atlassian.net/wiki/rest/calendar-services/1.0/calendar/subcalendar/privateurl/1c31d896-1906-4231-bf07-ec750601aa50?_=1452554155052"
• mycalendar.action:1 returns an error: "Uncaught SyntaxError: Unexpected token <"

I also tried "Export to iCalendar", which did work (but won't update when the team calendar is updated).