Subscribe to Webcal always gives error "Sorry we are unable to process request"

Subscribe to Webcal always gives error "Sorry we are unable to process request"

Any time I use Calendar to try to subscribe to a webcal I get error:

"Sorry we are unable to process your request.  Try again later."

It happens every time, with every webcal URL.

Here is one URL I try to subscribe to, webcal://mlb.am/tix/cubs_schedule_full

I am going to Calendar -> Settings ->Subscribe to, then pasting in the URL, and hitting subscribe.

Am I doing something wrong ?  Does the Subscribe To feature work ?