To be simple, consider anti spark loop key as a switch. You only instal it on the positive side / negative side.
So both wires that goes in and out of the anti spark should be the same color (only red or only black).
ohh I see you use the antiloop for the series connector. Then @lowGuido is correct, you have missed something within your balance plug.
If it still shorts, my suggestion is now to swap your balance plug around. 6s and 6s into 12s.
Left 6s balance plug to the right side and the right 6s balance plug to the left side. Seems weird at beginning (they are still end up as 12s anyway) but it does change the polarity of the balance plug removing your shorts.