Ey @Stephen Rose are there any known issues with the last bits released for the NGSC for MAC that add support to SPO Doc Libraries? A customer of mine has been strugling trying to sync a doc library with the last bits and it was no possible...we ended opening a support ticket that adviced us to install the december version of the sync client and run some configuration commands in MAC....and it worked!
It is critical that they flip the switch in SharePoint to allow the sync to happen. So, a) did they do so? b) do you see the sync button in a SharePoint library? c) If yes, what happens when you click sync?