The calendar was working fine until recently (a day or two) - now it is not working, and when I try to remove and add again, I get the following error message:
I also tried to add the calendar to my Google account's calendar, and it allowed me to add the link, but no planned workouts ever appeared (and I have a few).
We have pushed a new software version of 5.21, go ahead and return to the calendar and select to Publish again for a new URL that won't be affected by the issue.
If you still see an error after trying…
We are looking into this error that you are seeing, once we have an update from our team we will reply in this thread!
I have the same issue and can confirm it doesn't work with the calendar app on iOS, macOS or Google Calendar.
I unpublished and republished the calendar in Garmin Connect and tried to readd it to all…
Same Issue.
My calendar was subscribed since 2 years. I noticed that it was not updated since 2 days, so I tried to remove e re add and it's not working
Workaround for me: share with an office365 account which share again ... 
