Just saw your edit:Your DI is broken, it seems. You should start conversing with a JVC customer service or better through your dealer.
Edit. Just to be fool proof, so you choose the manual option among the Auto 1, Auto 2 and Manual, which then "un-greys" the -15 to 0 slider. And you then proceed to move the slider between -15 to 0. And you hear no motorized sounds and no change of brightness, correct?
Precisely. One has to be in Manual aperture mode in order to move the aperture slider. The slider responds to the remote, shifting from 0 to -15, but the aperture itself doesn't change. I ran back and forth through the range from 0 to -15, multiple times, in the hopes of 'unjamming' it, but it remains at the same aperture.