Ok, so first of all remove the atlas reference. This is just a guess (an educated one mind you

), but the atlas system appears to be a left-over of an older implementation of the buttons. When you remove the atlas reference don't forget to remove the comma in front of it. As for the comma at the very beginning, it seems to actually matter to the game if it's there or not (it works in both cases but slightly differently). So basically, try both ways, with comma and without the comma, but I think removing the comma is the more correct way so if they both produce the same result leave the comma out for now.
So am I to understand that you indeed generate mipmaps? First of all I'm having trouble with this because a 64x64 button will at most generate 5 mipmaps, and usually it's 4. However, regardless you should not be generating mipmaps for buttons, there's no need. So if you can disable that option, do and see if it helps.
Finally, if nothing works, try one of my buttons (you can grab one of the WH40k tech buttons in my sig) and see if it makes a difference. I generate them in Photoshop so it might be just your converter.