If we are talking MS-Outlook, then one reply on a Forum was to create a new folder based on Calendar and copy all existing Calendar items to it, delete the existing Calendar items in Calendar Folder and sync to this now empty folder. Introduce the items back into the old empty Calender folder and see if this cures the problem.
I still have problems syncing WMDC and my X50v. Email is fine but Calendar, Notes and Tasks, not a hope! It worked once with Notes but then failed to sync thereafter...
Regards, Nick.