I have encountered the same issue with raspbian stretch, plenty of free space, trying to sync small folder with several files.
Linux raspberrypi 4.14.70+ #1144 Tue Sep 18 17:20:50 BST 2018 armv6l GNU/Linux
Filesystem Size Used Avail Use% Mounted on
/dev/root 30G 1.1G 27G 4% /