Do you get this iOS Calendar error popup constantly? The error message says: “Calendar Invitation. Your response to the invitation cannot be sent” (see the image above). You tap OK but a few minutes later this pops up again? It is highly likely that you are getting this error because you responded (accept or reject) to a calendar invitation, and your response, for some reasons, could not be sent and thus you keep getting this notification.
This article explains how you can troubleshoot this error. If you are having this issue as well, this article will help you fix it:
Click here for steps to fix the Calendar Invitation Error