I just found a solution to this issue - after the update of DSM to version 6.0.2-8451, the "admin" account was disabled what caused BitTorrent Sync unable to run.
Solution: Open Control Panel -> Users -> Double-click on "admin" -> Uncheck "Disable this account" -> Go back to package center and activate the BitTorrent Sync -> now it should work
[Solved] Synology DSM 6.0.2-8451
in Sync for NAS (Network Attached Storage)
Posted
Thank you, solved my problem.