2) Human error involving reset of block systems. No idea how this process is undertaken, I hope this isn't possible from the op booth alone, as there isn't enough CCTV coverage to make this feasible.
Dare I say it, but i'm thinking human error too. The fact the one train had stopped on the lift hill indicated that the safety system had cut in and prevented it carrying on. Given the ride was having tech problems all day, engineers probably were there and not aware that an empty train had been sent out and probably thought the "red light" was another glitch and then decided to override the system.