iTunes could not back up the iPhone " " because a session could not be started with the iPhone

I have an iPhone 6 - it was brand new in July as my previous iPhone 6 broke and was replaced

I have iOS 8.4

I have the latest version of iTunes and OS X

However no matter what I do - I cannot get passed this message and get to update the iOS or indeed back up my phone

Does anyone have any clues?

Thanks!