freejp,
Let me see if I have this correct. In your first picture, going from left to right, we have:
1. Below the white box - goes to the Far Pool Light via the white box shown in the second picture.
2. Below the black box - goes to the Spa Light.
3. Below the black box - goes to the Near Pool Light.
(Items 2 and 3 above might be reversed).
If I have the above items correct, then I would think that the place to first check is not in the black box, but in the white box of your first picture. In this white box, we should have one black 120 Volt wire from the AquaLogic for the Spa Light and we should have one black 120 Volt wire from the AquaLogic for the Pool Lights (I am ignoring the white neutrals and green grounds for now). I would expect the one black wire for the Pool Lights to be connected via wirenuts to two other black wires - one of which goes down into the PVC conduit to the Far Pool Light and the other which goes up into the black box and down the copper conduit to the Near Pool Light. The black wire fom the AquaLogic for the Spa Light should either go directly up to the black box, or perhaps be connected by wirenut to only one other black wire, wit this other black wire going up to the black box.
If the above is not correct, then I am all messed up, and you should probably take pictures of the interior of the white and black box from the first picture and post them here at TFP.
If the above is correct, then you should be able to take off the wirenut connecting the three black wires together (being absolutely certain that the power is disconnected before doing so). With all three wires hanging free in the air (or, probably safer, temporarily placing a wirenut over each of the bare ends), turn back on the power and see if the GFI receptacle stays energized without tripping (which is what we would expect).
Now, after turning the power back off, connect the incoming black wire from the AquaLogic Pool Lights to only ONE of the two black wires going to the Pool Lights. Turn the power back on, and see what happens. Now turn off the power, and connect the incoming black wire from the AquaLogic to the OTHER Pool Light black wire, and turn the power back on. We would expect that the GFI receptacle would only trip while connected to one of the pool lights while not tripping while connected to the other pool light. This will tell you which pool light is being the problem child.
Hopefully I have not made any errors while writing the above. If this doesn't make sense, please ask. It is very possible that I made a mistake or skipped a necessary step.
Let us know what happens.
Titanium