Calendar sync not working.

My iphone 4 and Outlook Pro Plus 2010 were happily working together since I bought the phone about a year ago.

Suddenly though, the calendar is no longer able to sync with Outlook and it sticks at the calendar part of the sync process in itunes.

I can see that I am not alone in this respect, as there are lots of posts about this, but so far I haven't seen any sensible replies. I tried uninstalling itunes and everything Apple and then reinstalling but the problem persists. I tried computer back up, icloud back up and am now even trying wifi instead of itunes.

If I remove the calendars then the rest seems to work just fine. But of course, the iphone without a calendar is just a phone with a few games on it.

Does anyone have the answer?