The AFCI is designed to detect two types of fault. Series fault and Parallel fault. Series fault can be caused by a loose screw or back stab on a device or a bad wire nut. These are bad but they are mostly contained in j-boxes. a parallel fault is caused by directly shorting the hot and neutral (or ground) from pulling wires against staples, boxes, pounding nails in wrong places, etc.. these are worse because they are not usually found in the confines of a j-box.
An AFCI is not supposed to trip with the absence of a load.
Possibility: If the (properly functioning) AFCI is tripping with no load, you have a parallel fault. the electrical potential skips across the fault to complete the circuit. if it is a high resistance fault and you give it a light load, the load may have a lower resistance than the fault so the circuit works properly until there is a fluctuation in the voltage- then the power flows across the fault again.
Remedy: Check each box for missing tape or wire nuts. examine the boxes for electrical arcing if they are metal boxes. Tighten the side screws on each device and wrap the device with two layers of electrical tape. This can stop arcing between the hot of one outlet and the neutral of another in quad outlets, and the hot to the box in metal boxes. Also, check the insulation for nicks and missing sections. If you find any, wrap in at least two layers of electrical tape or replace wit pigtail or cut short if the wire is long enough. If possible inspect the run in an attic or basement/crawl space.
Also check again to make sure that there is truly no load on the line. there may be an outlet that you forgot that something is plugged in , or door bell transformer or humidifier, etc.. Disconnect from panel completely and ohms it at the source.
When you tried the standard breaker, did you remember to put the neutral back in the bar? This could account for the circuit not working at all...