problem with high resolution rendering

I have a problem rendering a high resolution image. I want to render a view from space onto the alps and I need it in a resolution of 5120 x 3200. I have a heightmap map texture, that creates shades around the mountains and an earthquake scenario using the map to normal feature for the texture. As soon as I turn OSA on (I tried 5 8 11 16, makes no difference), the shades almost vanish. This doesn’t happen when rendering with smaller resolutions. Do you know the cause of this problem? How can I solve it?

The first picture (only a small part) is rendered with OSA 5, the second without. (both 5120 x 3200)


did you turn off mipmaps?

yes, I turned mipmaps on and off in the map image options and it didn’t help.
Does the OSA maybe work differently for different resolutions?