Over the past few days I’ve received a few reports of ICS Calendar coming up empty when using iCloud (Apple) calendars.
When investigating the issue, I’m seeing that it’s also not possible to just subscribe to those URLs with the Mac Calendar app. In both cases, there’s a secure connection error, and a subsequent redirect loop.
It appears the issue has to do with some server configuration changes on Apple’s side. And the fix is to stop your original share, create a new share, and then replace the old ICS feed URL in your shortcode with the one generated by this new share.
Steps on setting up iCloud calendar sharing (be sure to share publicly) can be found here: https://support.apple.com/guide/icloud/share-a-calendar-mm6b1a9479/icloud