There is a control cable from the buckle to the inertia reel. This cable is supposed to move in and out as the seatbelt is buckled and un-buckled. Its purpose is to lock the inertia wheel when the belt is buckled.

My seatbelt was jammed because the end of the control cable was binding within the belt buckle. A tiny rubber seal had become dislocated within the buckle. I discarded this rubber seal so that now the cable travels smoothly. The seatbelt is no longer jammed.