A Microsoft file hosting and synchronization service.
I've been suffering this problem since June and had hoped that as it's part of a paid for 365 subscription, it would've been fixed with some urgency. My wife and I had been using shared folders synchronised across our respective PCs, We've now moved to Google Drive, as the 15GB each on the free service is sufficient for our needs. We'll also be reviewing whether to continue our subscription, as the functional updates to Office 365 are minimal and irritating!