There's something wrong with Vray's Alpha channel

[paulzet]

For some strange reason, Vray is now putting part of my building onto the alpha channel.

 

This is what the render looks like in the frame buffer:

 

But when I save it to a PNG with alpha channel, it comes out like this:

 

So I look at the alpha channel during render.

When it starts out, it looks like this:

 

Then as it continues, it looks like this:

 

What is happening to the building geometry? How do I fix this?