Since upgrading to iOS8, I've had constant problems with the Calendar app. First it started only syncing to the iCloud, and ignored everything from Exchange. I managed to stop this, and "delete all" entries from the Calendar, but no it isn't doing anything at all. The + symbol to create a new event is grayed out. I checked the settings, and Exchange is the default account. How do I get this working again?
If you have not already tried it, deleting the Exchange account from the iDevice (and all of the associated contacts, calendars and email if any was being synced), and then recreating the account can sometimes fix a variety of errors. Of course, making sure you know all of the server names and passwords and other needed information to set up the account would be wise before deleting the account.