The problem could be the lens, or the shutter, either way Schneider would be the ones to fix it, unless the actual Cambo mount has a problem. But Cambo should check the lens first IMO.
If you have a good relationship with a dealer, they can contact Cambo, send the lens to them, and then Cambo can send it to Schneider.
I assume this lens has a copal 0 shutter? If so, then either way Schneider will need to repair the lens as if they have to fix an element, then they will need to re-colimate the lens. However the 28mm has a huge outer element, i.e weight, and the weak link is the copal shutter. The shutter can be the problem, totally and if so, the shutter will need to be replaced. The 28XL and 32mm Rodenstock both share this weakness with the shutters due to the huge outer elements and the forward weight on the shutter. The copal can't take that much stress over time.
Capture Integration in Atlanta GA has a good relationship with Cambo. I am not sure where you are located, US or outside the US, but they might be a good place to start.
Paul C