Calendar Response Can't Be Sent

At some point I replied to a calendar event when I was offline. For about a month now I keep getting a pop up that my "Calendar Invitation: Your response to the invitation cannot be sent." It probably appears 10 or 12 times throughout the day and when I pick up my phone in the morning there are 7 or 8 of them that pop up at once and I have to respond to each of them before it will let me do anything.

How do I get these notifications to stop? I don't which event it was so I can't go back and change or resubmit my response or delete the event.

I've upgrade the iOS, force restarted the calendar app, restarted the phone. Nothing works.

Please help! This is maddening.