For me the new 1.4.103 on my QNAP 459 seems to work fine. At least there are no more crashes of BT Sync on the QNAP. I will never update BT Sync on a QNAP again ;-)
It's the same here with TS-459 Pro II, BT Sync stops working on the NAS after a some seconds or a few minutes. Is it possible to install a downgrade to version before? Are there somewhere instructions for that?