My animated book shows half a black page for some reason. What is going on?

So I made this simple rig for thi![alt text][1]s book model:

3 to manage the cover deformation, and one more for the page deformation as shown in the screen. In Max it looks fine, but in Unity half my left page is black for some reason.

![alt text][2]

2 days wasted on this and still nowhere near a solution. I have tried everything, my normals are fine, my geo is fine. I know this is linked to my 4th bone ("additional bone" in that screen) cause as soon as I remove it from the model the error is gone - but then so is my page deformation. I am at my wits end, anyone can push me in the right direction? [1]: /storage/temp/40900-untitled-5.jpg [2]: /storage/temp/40901-untitled-2.jpg