+1 for what buzz said.
The certifier 'should' pickup on issues with water ingress to the cable - it would fail it if it was bad enough to cause a problem.
I agree the most likely thing is a ground loop. And this would also explain why it worked for so long and now has problems. Ground paths and resistance change in homes sometimes. Could be a corroded connection somewhere in the electrical (especially out at the pool house). Or it could be some recent changes someone did, that has caused the issue in the electrical.

To test, run a 100ft cat cable across the ground from the pool house to the switch in the main house and see what happens. Or even 200ft if you have to take a twisted path to get from one to the other.