The overload warning is really only triggered by the sensor circuitry detecting an incorrect load across the speaker outputs, so its a case of checking that none of the speaker connections has shorted anywhere. This fault can also apply if another speaker has been added to the system, or of one of the originals substituted for another type of a lower impedance. (which should not be done!) If by any chance none of these applies then the amplifier I/C associated with the surround output is very likely to be faulty.