Help, why do I have a black shade on my groundglass?

hi everyone,

If I am looking at my groundglass, I am seeing a black stroke of shade on both the right and left side of the image.
Why is that?
If I am using shifts to left or right, the shade will disapear on one side...
I can not imagine it's the imagecircle, since no movements have been made.

Hopefully someone can help me out here, THANKS!

kind regards game (lens used is 150mm f5.6 rodenstock apo-sironar)

It sounds like the bellows is impinging on the image. Try gathering the bellows toward the front standard.