Subscribe to Calendar not working

I have a calendar with a small number of events. The only repeating events are using ‘Custom’ dates with only 1 – 3 repetitions.

The ‘Subscribe to Calendar’ function does not work with any calendar on the list. Our clients are trying to use Outlook and are complaining that it’s not working (I don’t use Outlook and they haven’t said what the error message says). Google+ says “The address that you provided did not contain a calendar in a valid iCal or GData format”, while iCal says ‘The data downloaded from webcal://…. isn’t valid’.

With iCal, if I take the “webcal://…” URL, paste it into the address bar and change “webcal://” to “https://”, the calendar successfully populates with all the events. However, we need it to work properly with the button for our clients.

Any ideas what might be going on here and how to fix it?