I've removed and re-added the folder, and it seems to be working now.
What I suspect is that since the latest update, for some unknown reason the BitSync has been entirely unable to write anything files to the SD card. Had to remove and re-add all folders, and hopefully this is now working again. If not, will have to down-grade my Android version.
Also, when adding folders in the latest Android App using the QR code, the target location is set to: SD:///'FOLDERNAME', which promptly results in a 'directory not found' upon sync start. The only way around this at the moment is to manually select the folder.
This is a bug and should be fixed.