DIAGNOSIS Output synchronization failure. One generator's inverter board is detecting a phase mismatch and shutting down for protection.
FIX Start both generators separately first. Make sure both are running steady with Eco Mode OFF. Plug the parallel cables into the *generators* first. Then plug the cables into each other. Finally, turn on the parallel switch on *both* control panels. The order matters.
IF THAT DIDN'T WORK One of the parallel ports on a generator might be damaged. Inspect the pins inside the port for bending or corrosion. A bent pin will prevent proper communication.
HEADS UP The generators must be the same model (iGen4500 to iGen4500) for parallel to work correctly. Even a different year model can have firmware differences that cause this.