You get this message when you have a calendar subscription which is not being updated anymore.
We suggest you remove that subscription from your calendar application.
It saves bandwidth, battery power and all those good things.
To remove this calendar
- Press the (i) sign to see which Calendar it involves
- Go to iOS device Settings > Calendar
(or Mail, Contacts, Calendars in iOS9 and below)
- In the Subscribed Calendars part of the Accounts section select the calendar you want to delete
- Click 'Delete Account' to unsubscribe from this calendar