As well as showing when a channel is clipping, the Overload LED performs a second function. When the LED is lit continuously, it means that this channel's level is being displayed on the front panel meter. The channels can be cycled through by pressing the "Source" button on the front panel of the unit.
Because of this, it is perfectly normal to have one of the Overload LEDs constantly lit at all times. This is not indicative of a hardware fault with the unit.