AFCI breaker trouble?
AFCIs trip on ground faults too

Most AFCI breakers require the circuit neutral to be connected to the breaker like a GFCI breaker, not to the panelboard neutral bar.  This is done so that the AFCI can correctly detect certain classes of faults (such as a neutral to ground arc), but also gives them properties similar to a GFCI in that they will trip on the same wiring errors that trip GFCIs (such as improperly wired or shared neutrals), and will do so as soon as load is applied in the case of an errant neutral wire.   Fixing the neutral wiring will make them happy again.