Troubleshooting arc fault breaker
Since you have removed the loads and replaced the breaker there could be a pinched or skined wire in this circuit causing the fault. A megger to verify the insulation is good is the quickest way I know. The wires are removed from the breaker and all switches off. If there is a low resistance at less than 500v there is usually a staple or nail through the insulation. If the test is good start turning on switches, lamps/ bulbs out when one circuit shows a low value this is the one to check. With all this said if you have recently added flouresent lighting, a large computer / UPS or a large dimmer on any kind of lighting circuit arc fault breakers can't tell the difference from an arc and a electronic load and will trip.