I saw this message over the course of this week coming into my mailbox. No one else in the company was affected just me. I saw it on an Exchange 2007 server that was running on SBS 2008.
It also came back in a number of other formats including
Synchronization with your WP failed for 3 items
Synchronization with your iPad failed for 3 items
Synchronization with your WindowsMail failed for 3 items
The body of the message went on to show
as well as the appointments it had failed to synchronise.
I could not figure out what it was at first and then realised that the appointments in question all had a new status of “Working Elsewhere” which is new to Office 2013. All this started when I upgraded to Office 2013 and then started to use these new status to describe that I was working on a customer site vs just out of the office.
I figured that these devices or ActiveSync does not know how to handle the new status and therefore it gives the error. Once I reverted my appointments back to Out of office, it worked just fine.