A Microsoft file hosting and synchronization service.
The next phase of the 'update' appears to have happened for me last night - OneDrive (Microsoft) has deleted the links to the shared folders that were put in place of the synchronised folders so that I now have no access to the shared files from Explorer. The Internet shortcuts are now sitting in the Recycle Bin while OneDrive tries to process the [two] changes - it has been sat for hours today [not] doing that ...
... and, after two weeks apparently doing nothing, my shared folders are back within Explorer as they should be.
Well, almost, since the files that were permanently resident on my hard drive were now directly accessible on the cloud - just another 8GB synchronisation to get things back properly.
So, in conclusion, Microsoft broke it and are quietly and slowly, without admitting anything, fixing it - we are only users / customers after all ...