Ok, I found a problem in the LS-Shader. If I use a realtime-mirror, then the LightShafts doesn't work correctly: The screensize is scaled to the size of the realtime-mirror-view and I see only the full sun (no raysplitting at objects).

I used the predefined mirror from gamestudio.

But I can't find the differences in the shaders. The mirror is rendering in an extra-view and to the model (floor, water...), and the LS renders to the camera.