Where is this white coming from?
TL;DR An Iray section plane node causes a dress' interior to turn white. Where is it coming from?
I have a scene and I pulled the camera back through the ceiling. So I created a iray section plane node, set it to block light, put it just below the ceiling, made the ceiling invisible in viewport and went on with what I was doing.
Turned on IRAY preview and there's this white showing on the interior of this model's dress: https://www.daz3d.com/dforce-willow-dress-for-genesis-9
The white also appears when I do a full render.
I switched to perspective view, which was just pointing head on to the figure, everything is fine. When the perspective view crossed the iray section plane node, the white appeared again. Hide the IRAY section plane node and the white goes away. If I move the camera so that the IRAY node is not longer between it and the figure, the white goes away. Hiding the node, the white goes away. It is definitely the IRAY node that is causing that texture to turn white. (Please don't tell me the dress is colliding with the chair. I know this. I've hidden the chair and it doesn't change how the texture renders.)
I tried deleting the dress and recreating it. Same issue. Where is this white coming from? I've included part the Glossy Color part of the texture for reference. I've also hidden the figure to show the white interior shows up in the entirety of the dress. Exiting and restarting Daz didn't change anything.
I can render the scene without the iray section plane node, but that is sub-optimal.
Any suggestions on what to look at would help. But, as long as the camera (or perspective view) passes through this node, the white appears.

Comments
Can't really imagine what can do that.
What is the section plane blocking?
What shader does it say on the dress?
have you got a white background and rendering jpg?
If you have no background (and sky/dome not visible in render) and render png is it transparent?
has the dress got an iray mat script applied?
The plane is allowing me to move the camera through the ceiling.
Since folks asked, this is the surface data for the dress. But, as I said, it is the stock dress asset from the store item with a store provided material applied to it.
What happens if you turn 'Thin walled' On in the dress surface settings?
At a guess it looks like your section plain is messing with Iray's ability to flip normals to face the camera, as those white areas are the backfaces of the mesh.
But why? And how can that be fixed?
This did work. The white is gone when Thin Walled is On.