No longer able to subscribe to Facebook calendar

TL;DR: iOS 17.0.3 Calendar app refuses to subscribe to Facebook ICS content.

After about 10+ years of being able to subscribe to my Facebook events on my iPhone, it has recently stopped working.

From https://www.facebook.com/events/calendar there is an "Add to calendar" button. This returns a URL looking like https://www.facebook.com/events/ical/upcoming/?uid=111111111&key=aaaaaaaaaaaaaaaa The URL format has been unchanged for many years. And I can add my URL to e.g., Google Calendar without problems.

The URL did work earlier this year as well. But at some point, it has stopped working. There is no difference whether I try to add the URL directly in the Calendar app or via the Add Subscribed Calendar option in the Settings app.

When I try to re-add the URL, I first get the dialog:

Insecure Connection

The connection is not secure. Do you want to continue subscribing?

Cancel / Subscribe

When selecting Subscribe, an error message shows "Validation failed. Please edit the URL and try again."

I have seen suggested workarounds of adding the Facebook URL to Google Calendar (that works) and then adding the Google Calendar URL in the Calendar app. That scenario presents me with the same error that the connection is insecure.

I have several other calendars that I can subscribe to without any issues. And where it gets even more odd, is that if I navigate to https://www.facebook.com/events/calendar in Firefox on the phone, download the ics file, share the ics file to the Files app, and then open that file it the Files app on the phone, it does show the calendar content correctly.

Something in the Calendar app appear to refuse to correctly read the Facebook ICS content. The same content that it can read if it exists as a file on the device.