Open your calendar app, and see if you have a subscribed calendar listed that you do not expect and do not need. If you do have an unexpected and unneeded subscribed calendar, remove it, and see if that clears this error.
Delete spam calendars and events on iPhone – Apple Support (UK)
Some of these cases are due to websites and sketchy ad networks that are offering notifications, and—should the offered calendar be accepted by the user—more than a few of these offered calendars are filled with spam.
If you’re on iOS 15 on an iPhone X, I’d suggest an update to iOS 16, too.