The likely issue is a fault in the aperture control mechanism, either of the lens or camera. There is a mechanical linkage there, where a small lever in the camera pushes a flat metal lever on the lens to adjust aperture. If you have tried multiple lenses the likely culprit is the camera. If you want to check, set a long exposure (so you can see better), on manual, set the aperture to small (say F16 or so) and expose, then look down the lens, and see if the aperture blades close. If they do not, take the lens off and move the lever in the lens by hand (it is the reverse there, where you move it to open due to spring loading). If it moves smoothly on the lens, the camera needs repair. The camera's control mechanism does not need to move when the aperture is wide open, which is likely why the error only occurs when stopped down.
If the control mechanism is failing to close the iris, it is likely in the mechanism itself (a hugely complex looking mass of gears). If the mechanism is failing to open the iris (i.e. on mounting the lens the blades remain closed as you twist the lens in) sometimes it can be because the lever in the camera is bent out of the way and not contacting the lens lever, and can be bent back. That does not appear to be your problem, and so I think only a repair shop can correct it by replacing the control module.