How do I fix red exclamation marks on shared Calendar?

I have a red exclamation next to all my shared calendars. They were working perfectly until about 2 weeks ago and now if I enter an event it’s not showing on shared devices and vice versa.

I’ve tried removing calendars and re adding them, rebooting device, deleting calendar app, signing out and back in to iCloud. But absolutely nothing is resolving it.

When you tap on the red exclamation mark it opens the calendar information but shows absolutely no information regarding the issue which is a terrible UX, so how am I supposed to know what’s causing this issue?

Any help or suggestions would be appreciated or iCal will have to be replaced!

[Re-Titled by Moderator]

Original Title: Calendar errors