Hi,
My answer may only apply to people with the developmental version of Mendeley. EndNote technical support just resolved this on my PC with Windows 7, Word 2013, and the developmental version of Mendeley (a version that is not distributed to the public except by signing up for it). The app add in was what seemed to be causing the conflict. In my version of Word the place to turn it off is Word File menu + Options + Add-Ins+ in the Manage list at the foot of the window go to Word Add-ins and unselect the Mendeley developmental add-in. I’ve had the Mendeley add-in for months and never saw this before, so it may just be with the most recent developmental version or versions.
Have a nice day
John