Look what one stiff link in chain can do 
Rivet in that link was just a little too tight and once link passed over sprocket, tension force on the slack side was not enough to straight it out. When motor started to turn in other direction, the bended link stick to sprocket and caused that:
Lesson learnt - before any new installation some kind of pre stretching/bending of chain is recommended. Using sprocket hang somewhere overhead of your garage/shop and 5 - 10 kg weights attached to chain should do the trick. It should be be run several times on whole lenght - on both sides (pre bend in both direction of bending).
Tip: small 8mm ball bearing available in any hobby shop with significantly reduce friction in the sprocket/hook arrangement.
(sorry for blurred picture)
