The GM integrated brake system is looking for a low-resistance, high-current path to ground through the magnets in the electric brakes. Since the EB is a single-axle trailer, it will not take much of a poor connection to make the controller see too high a resistance. This is less likely on a tandem trailer with 4 magnets in parallel.
As mentioned above, really check the ground and brake pins in the umbilical. They are the two pins exactly opposite the raised key slot on the plug.
I have heard of cases with single axles where an additional dummy resistor needed to be added at the trailer end to insure that the accumulated resistance is low enough to satisfy the integrated controller.
John W. Irwin
2014 Touring Coach, "Sabre-Dog IV"